From 54651a71682c52917acab1110fe55bc6a07bd0f3 Mon Sep 17 00:00:00 2001
From: Craig Tiller <ctiller@google.com>
Date: Thu, 10 Aug 2023 09:40:13 -0700
Subject: [PATCH] [promises] New `Seq`, `TrySeq` implementation (#33991)

Our current implementation of `Seq`, `TrySeq` leverage some complicated
template stuff to work, which makes them hard to maintain. I've been
thinking about ways to simplify that for some time and had something
like this in mind - using a code generator that's at least a little more
understandable to code generate most of the complexity into a file that
is checkable.

Concurrently - I have a cool optimization in mind - but it requires that
we can move promises after polling, which is a contract change. I'm
going to work through the set of primitives we have in the coming weeks
and change that contract to enable the optimization.

---------

Co-authored-by: ctiller <ctiller@users.noreply.github.com>
---
 BUILD                                         |    2 -
 CMakeLists.txt                                |    6 -
 Package.swift                                 |    2 +-
 build_autogenerated.yaml                      |   34 +-
 gRPC-C++.podspec                              |    4 +-
 gRPC-Core.podspec                             |    4 +-
 grpc.gemspec                                  |    2 +-
 package.xml                                   |    2 +-
 src/core/BUILD                                |   24 +-
 .../channel_idle/channel_idle_filter.cc       |    1 +
 src/core/lib/channel/connected_channel.cc     |    1 -
 src/core/lib/channel/promise_based_filter.cc  |    2 +-
 src/core/lib/promise/detail/basic_seq.h       |  373 +--
 src/core/lib/promise/detail/seq_state.h       | 2076 +++++++++++++++++
 src/core/lib/promise/seq.h                    |   21 +-
 src/core/lib/promise/try_seq.h                |   36 +-
 src/core/lib/resource_quota/memory_quota.cc   |    1 -
 .../security/transport/client_auth_filter.cc  |    2 +-
 .../security/transport/server_auth_filter.cc  |    2 +
 src/core/lib/surface/call.cc                  |    1 -
 src/core/lib/surface/server.cc                |    2 +-
 test/core/filters/filter_test.cc              |    2 +-
 test/core/promise/BUILD                       |    3 -
 test/core/promise/for_each_test.cc            |    1 -
 test/core/promise/loop_test.cc                |    3 +-
 test/core/promise/map_pipe_test.cc            |    1 -
 test/core/promise/promise_fuzzer.cc           |    2 +
 test/core/promise/seq_test.cc                 |    2 +
 test/core/promise/try_seq_metadata_test.cc    |    2 +
 test/core/promise/try_seq_test.cc             |    1 +
 tools/codegen/core/gen_seq.py                 |  271 +++
 tools/doxygen/Doxyfile.c++.internal           |    2 +-
 tools/doxygen/Doxyfile.core.internal          |    2 +-
 33 files changed, 2457 insertions(+), 433 deletions(-)
 create mode 100644 src/core/lib/promise/detail/seq_state.h
 create mode 100755 tools/codegen/core/gen_seq.py

diff --git a/BUILD b/BUILD
index a1a9c395447..c7a7e844c43 100644
--- a/BUILD
+++ b/BUILD
@@ -1534,7 +1534,6 @@ grpc_cc_library(
         "//src/core:arena",
         "//src/core:arena_promise",
         "//src/core:atomic_utils",
-        "//src/core:basic_seq",
         "//src/core:bitset",
         "//src/core:cancel_callback",
         "//src/core:channel_args",
@@ -1799,7 +1798,6 @@ grpc_cc_library(
         "//src/core:activity",
         "//src/core:arena",
         "//src/core:arena_promise",
-        "//src/core:basic_seq",
         "//src/core:channel_args",
         "//src/core:channel_fwd",
         "//src/core:closure",
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 5b5e57d76ab..3e281c5c561 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -8777,7 +8777,6 @@ target_link_libraries(chunked_vector_test
   absl::hash
   absl::type_traits
   absl::statusor
-  absl::utility
   gpr
   upb
 )
@@ -12110,7 +12109,6 @@ target_link_libraries(flow_control_test
   absl::hash
   absl::type_traits
   absl::statusor
-  absl::utility
   gpr
   upb
 )
@@ -15292,7 +15290,6 @@ target_link_libraries(interceptor_list_test
   absl::hash
   absl::type_traits
   absl::statusor
-  absl::utility
   gpr
   upb
 )
@@ -16227,7 +16224,6 @@ target_link_libraries(loop_test
   ${_gRPC_ALLTARGETS_LIBRARIES}
   absl::type_traits
   absl::statusor
-  absl::utility
   gpr
 )
 
@@ -22151,7 +22147,6 @@ target_link_libraries(seq_test
   ${_gRPC_ZLIB_LIBRARIES}
   ${_gRPC_ALLTARGETS_LIBRARIES}
   absl::type_traits
-  absl::utility
   gpr
 )
 
@@ -26523,7 +26518,6 @@ target_link_libraries(try_seq_test
   ${_gRPC_ALLTARGETS_LIBRARIES}
   absl::type_traits
   absl::statusor
-  absl::utility
   gpr
 )
 
diff --git a/Package.swift b/Package.swift
index 56699d86e08..afeb98b7f1c 100644
--- a/Package.swift
+++ b/Package.swift
@@ -1447,8 +1447,8 @@ let package = Package(
         "src/core/lib/promise/detail/basic_seq.h",
         "src/core/lib/promise/detail/promise_factory.h",
         "src/core/lib/promise/detail/promise_like.h",
+        "src/core/lib/promise/detail/seq_state.h",
         "src/core/lib/promise/detail/status.h",
-        "src/core/lib/promise/detail/switch.h",
         "src/core/lib/promise/exec_ctx_wakeup_scheduler.h",
         "src/core/lib/promise/for_each.h",
         "src/core/lib/promise/if.h",
diff --git a/build_autogenerated.yaml b/build_autogenerated.yaml
index a21e36895b1..e780ca50f6d 100644
--- a/build_autogenerated.yaml
+++ b/build_autogenerated.yaml
@@ -857,8 +857,8 @@ libs:
   - src/core/lib/promise/detail/basic_seq.h
   - src/core/lib/promise/detail/promise_factory.h
   - src/core/lib/promise/detail/promise_like.h
+  - src/core/lib/promise/detail/seq_state.h
   - src/core/lib/promise/detail/status.h
-  - src/core/lib/promise/detail/switch.h
   - src/core/lib/promise/exec_ctx_wakeup_scheduler.h
   - src/core/lib/promise/for_each.h
   - src/core/lib/promise/if.h
@@ -2253,8 +2253,8 @@ libs:
   - src/core/lib/promise/detail/basic_seq.h
   - src/core/lib/promise/detail/promise_factory.h
   - src/core/lib/promise/detail/promise_like.h
+  - src/core/lib/promise/detail/seq_state.h
   - src/core/lib/promise/detail/status.h
-  - src/core/lib/promise/detail/switch.h
   - src/core/lib/promise/exec_ctx_wakeup_scheduler.h
   - src/core/lib/promise/for_each.h
   - src/core/lib/promise/if.h
@@ -3754,8 +3754,8 @@ libs:
   - src/core/lib/promise/detail/basic_seq.h
   - src/core/lib/promise/detail/promise_factory.h
   - src/core/lib/promise/detail/promise_like.h
+  - src/core/lib/promise/detail/seq_state.h
   - src/core/lib/promise/detail/status.h
-  - src/core/lib/promise/detail/switch.h
   - src/core/lib/promise/exec_ctx_wakeup_scheduler.h
   - src/core/lib/promise/for_each.h
   - src/core/lib/promise/if.h
@@ -4307,8 +4307,8 @@ targets:
   - src/core/lib/promise/detail/basic_seq.h
   - src/core/lib/promise/detail/promise_factory.h
   - src/core/lib/promise/detail/promise_like.h
+  - src/core/lib/promise/detail/seq_state.h
   - src/core/lib/promise/detail/status.h
-  - src/core/lib/promise/detail/switch.h
   - src/core/lib/promise/join.h
   - src/core/lib/promise/poll.h
   - src/core/lib/promise/promise.h
@@ -6115,8 +6115,8 @@ targets:
   - src/core/lib/promise/detail/basic_seq.h
   - src/core/lib/promise/detail/promise_factory.h
   - src/core/lib/promise/detail/promise_like.h
+  - src/core/lib/promise/detail/seq_state.h
   - src/core/lib/promise/detail/status.h
-  - src/core/lib/promise/detail/switch.h
   - src/core/lib/promise/exec_ctx_wakeup_scheduler.h
   - src/core/lib/promise/loop.h
   - src/core/lib/promise/map.h
@@ -6167,7 +6167,6 @@ targets:
   - absl/hash:hash
   - absl/meta:type_traits
   - absl/status:statusor
-  - absl/utility:utility
   - gpr
   - upb
   uses_polling: false
@@ -7761,8 +7760,8 @@ targets:
   - src/core/lib/promise/detail/basic_seq.h
   - src/core/lib/promise/detail/promise_factory.h
   - src/core/lib/promise/detail/promise_like.h
+  - src/core/lib/promise/detail/seq_state.h
   - src/core/lib/promise/detail/status.h
-  - src/core/lib/promise/detail/switch.h
   - src/core/lib/promise/exec_ctx_wakeup_scheduler.h
   - src/core/lib/promise/loop.h
   - src/core/lib/promise/map.h
@@ -7818,7 +7817,6 @@ targets:
   - absl/hash:hash
   - absl/meta:type_traits
   - absl/status:statusor
-  - absl/utility:utility
   - gpr
   - upb
   uses_polling: false
@@ -7854,8 +7852,8 @@ targets:
   - src/core/lib/promise/detail/basic_seq.h
   - src/core/lib/promise/detail/promise_factory.h
   - src/core/lib/promise/detail/promise_like.h
+  - src/core/lib/promise/detail/seq_state.h
   - src/core/lib/promise/detail/status.h
-  - src/core/lib/promise/detail/switch.h
   - src/core/lib/promise/exec_ctx_wakeup_scheduler.h
   - src/core/lib/promise/for_each.h
   - src/core/lib/promise/if.h
@@ -8226,8 +8224,8 @@ targets:
   - src/core/lib/promise/detail/basic_seq.h
   - src/core/lib/promise/detail/promise_factory.h
   - src/core/lib/promise/detail/promise_like.h
+  - src/core/lib/promise/detail/seq_state.h
   - src/core/lib/promise/detail/status.h
-  - src/core/lib/promise/detail/switch.h
   - src/core/lib/promise/exec_ctx_wakeup_scheduler.h
   - src/core/lib/promise/for_each.h
   - src/core/lib/promise/if.h
@@ -9702,8 +9700,8 @@ targets:
   - src/core/lib/promise/detail/basic_seq.h
   - src/core/lib/promise/detail/promise_factory.h
   - src/core/lib/promise/detail/promise_like.h
+  - src/core/lib/promise/detail/seq_state.h
   - src/core/lib/promise/detail/status.h
-  - src/core/lib/promise/detail/switch.h
   - src/core/lib/promise/exec_ctx_wakeup_scheduler.h
   - src/core/lib/promise/interceptor_list.h
   - src/core/lib/promise/loop.h
@@ -9758,7 +9756,6 @@ targets:
   - absl/hash:hash
   - absl/meta:type_traits
   - absl/status:statusor
-  - absl/utility:utility
   - gpr
   - upb
   uses_polling: false
@@ -10075,8 +10072,8 @@ targets:
   - src/core/lib/promise/detail/basic_seq.h
   - src/core/lib/promise/detail/promise_factory.h
   - src/core/lib/promise/detail/promise_like.h
+  - src/core/lib/promise/detail/seq_state.h
   - src/core/lib/promise/detail/status.h
-  - src/core/lib/promise/detail/switch.h
   - src/core/lib/promise/join.h
   - src/core/lib/promise/latch.h
   - src/core/lib/promise/poll.h
@@ -10179,7 +10176,7 @@ targets:
   - src/core/lib/promise/detail/basic_seq.h
   - src/core/lib/promise/detail/promise_factory.h
   - src/core/lib/promise/detail/promise_like.h
-  - src/core/lib/promise/detail/switch.h
+  - src/core/lib/promise/detail/seq_state.h
   - src/core/lib/promise/loop.h
   - src/core/lib/promise/poll.h
   - src/core/lib/promise/seq.h
@@ -10188,7 +10185,6 @@ targets:
   deps:
   - absl/meta:type_traits
   - absl/status:statusor
-  - absl/utility:utility
   - gpr
   uses_polling: false
 - name: map_pipe_test
@@ -10223,8 +10219,8 @@ targets:
   - src/core/lib/promise/detail/basic_seq.h
   - src/core/lib/promise/detail/promise_factory.h
   - src/core/lib/promise/detail/promise_like.h
+  - src/core/lib/promise/detail/seq_state.h
   - src/core/lib/promise/detail/status.h
-  - src/core/lib/promise/detail/switch.h
   - src/core/lib/promise/exec_ctx_wakeup_scheduler.h
   - src/core/lib/promise/for_each.h
   - src/core/lib/promise/if.h
@@ -13441,14 +13437,13 @@ targets:
   - src/core/lib/promise/detail/basic_seq.h
   - src/core/lib/promise/detail/promise_factory.h
   - src/core/lib/promise/detail/promise_like.h
-  - src/core/lib/promise/detail/switch.h
+  - src/core/lib/promise/detail/seq_state.h
   - src/core/lib/promise/poll.h
   - src/core/lib/promise/seq.h
   src:
   - test/core/promise/seq_test.cc
   deps:
   - absl/meta:type_traits
-  - absl/utility:utility
   - gpr
   uses_polling: false
 - name: sequential_connectivity_test
@@ -15414,8 +15409,8 @@ targets:
   - src/core/lib/promise/detail/basic_seq.h
   - src/core/lib/promise/detail/promise_factory.h
   - src/core/lib/promise/detail/promise_like.h
+  - src/core/lib/promise/detail/seq_state.h
   - src/core/lib/promise/detail/status.h
-  - src/core/lib/promise/detail/switch.h
   - src/core/lib/promise/poll.h
   - src/core/lib/promise/try_seq.h
   src:
@@ -15423,7 +15418,6 @@ targets:
   deps:
   - absl/meta:type_traits
   - absl/status:statusor
-  - absl/utility:utility
   - gpr
   uses_polling: false
 - name: unique_type_name_test
diff --git a/gRPC-C++.podspec b/gRPC-C++.podspec
index d9590738516..a9ad7359874 100644
--- a/gRPC-C++.podspec
+++ b/gRPC-C++.podspec
@@ -951,8 +951,8 @@ Pod::Spec.new do |s|
                       'src/core/lib/promise/detail/basic_seq.h',
                       'src/core/lib/promise/detail/promise_factory.h',
                       'src/core/lib/promise/detail/promise_like.h',
+                      'src/core/lib/promise/detail/seq_state.h',
                       'src/core/lib/promise/detail/status.h',
-                      'src/core/lib/promise/detail/switch.h',
                       'src/core/lib/promise/exec_ctx_wakeup_scheduler.h',
                       'src/core/lib/promise/for_each.h',
                       'src/core/lib/promise/if.h',
@@ -2001,8 +2001,8 @@ Pod::Spec.new do |s|
                               'src/core/lib/promise/detail/basic_seq.h',
                               'src/core/lib/promise/detail/promise_factory.h',
                               'src/core/lib/promise/detail/promise_like.h',
+                              'src/core/lib/promise/detail/seq_state.h',
                               'src/core/lib/promise/detail/status.h',
-                              'src/core/lib/promise/detail/switch.h',
                               'src/core/lib/promise/exec_ctx_wakeup_scheduler.h',
                               'src/core/lib/promise/for_each.h',
                               'src/core/lib/promise/if.h',
diff --git a/gRPC-Core.podspec b/gRPC-Core.podspec
index c6f4811efe8..8a59d4bd0ae 100644
--- a/gRPC-Core.podspec
+++ b/gRPC-Core.podspec
@@ -1548,8 +1548,8 @@ Pod::Spec.new do |s|
                       'src/core/lib/promise/detail/basic_seq.h',
                       'src/core/lib/promise/detail/promise_factory.h',
                       'src/core/lib/promise/detail/promise_like.h',
+                      'src/core/lib/promise/detail/seq_state.h',
                       'src/core/lib/promise/detail/status.h',
-                      'src/core/lib/promise/detail/switch.h',
                       'src/core/lib/promise/exec_ctx_wakeup_scheduler.h',
                       'src/core/lib/promise/for_each.h',
                       'src/core/lib/promise/if.h',
@@ -2737,8 +2737,8 @@ Pod::Spec.new do |s|
                               'src/core/lib/promise/detail/basic_seq.h',
                               'src/core/lib/promise/detail/promise_factory.h',
                               'src/core/lib/promise/detail/promise_like.h',
+                              'src/core/lib/promise/detail/seq_state.h',
                               'src/core/lib/promise/detail/status.h',
-                              'src/core/lib/promise/detail/switch.h',
                               'src/core/lib/promise/exec_ctx_wakeup_scheduler.h',
                               'src/core/lib/promise/for_each.h',
                               'src/core/lib/promise/if.h',
diff --git a/grpc.gemspec b/grpc.gemspec
index d3d4df3b6cc..c14953000cb 100644
--- a/grpc.gemspec
+++ b/grpc.gemspec
@@ -1453,8 +1453,8 @@ Gem::Specification.new do |s|
   s.files += %w( src/core/lib/promise/detail/basic_seq.h )
   s.files += %w( src/core/lib/promise/detail/promise_factory.h )
   s.files += %w( src/core/lib/promise/detail/promise_like.h )
+  s.files += %w( src/core/lib/promise/detail/seq_state.h )
   s.files += %w( src/core/lib/promise/detail/status.h )
-  s.files += %w( src/core/lib/promise/detail/switch.h )
   s.files += %w( src/core/lib/promise/exec_ctx_wakeup_scheduler.h )
   s.files += %w( src/core/lib/promise/for_each.h )
   s.files += %w( src/core/lib/promise/if.h )
diff --git a/package.xml b/package.xml
index 3a8afe9b3fd..e0cd73dc383 100644
--- a/package.xml
+++ b/package.xml
@@ -1435,8 +1435,8 @@
     <file baseinstalldir="/" name="src/core/lib/promise/detail/basic_seq.h" role="src" />
     <file baseinstalldir="/" name="src/core/lib/promise/detail/promise_factory.h" role="src" />
     <file baseinstalldir="/" name="src/core/lib/promise/detail/promise_like.h" role="src" />
+    <file baseinstalldir="/" name="src/core/lib/promise/detail/seq_state.h" role="src" />
     <file baseinstalldir="/" name="src/core/lib/promise/detail/status.h" role="src" />
-    <file baseinstalldir="/" name="src/core/lib/promise/detail/switch.h" role="src" />
     <file baseinstalldir="/" name="src/core/lib/promise/exec_ctx_wakeup_scheduler.h" role="src" />
     <file baseinstalldir="/" name="src/core/lib/promise/for_each.h" role="src" />
     <file baseinstalldir="/" name="src/core/lib/promise/if.h" role="src" />
diff --git a/src/core/BUILD b/src/core/BUILD
index cf94603020d..b6c7590a4fd 100644
--- a/src/core/BUILD
+++ b/src/core/BUILD
@@ -665,21 +665,30 @@ grpc_cc_library(
 
 grpc_cc_library(
     name = "basic_seq",
-    external_deps = [
-        "absl/meta:type_traits",
-        "absl/utility",
-    ],
     language = "c++",
     public_hdrs = [
         "lib/promise/detail/basic_seq.h",
     ],
+    deps = [
+        "construct_destruct",
+        "poll",
+        "//:gpr_platform",
+    ],
+)
+
+grpc_cc_library(
+    name = "seq_state",
+    external_deps = ["absl/base:core_headers"],
+    language = "c++",
+    public_hdrs = [
+        "lib/promise/detail/seq_state.h",
+    ],
     deps = [
         "construct_destruct",
         "poll",
         "promise_factory",
         "promise_like",
-        "switch",
-        "//:gpr_platform",
+        "//:gpr",
     ],
 )
 
@@ -693,6 +702,7 @@ grpc_cc_library(
         "basic_seq",
         "poll",
         "promise_like",
+        "seq_state",
         "//:gpr_platform",
     ],
 )
@@ -713,6 +723,7 @@ grpc_cc_library(
         "poll",
         "promise_like",
         "promise_status",
+        "seq_state",
         "//:gpr_platform",
     ],
 )
@@ -1080,7 +1091,6 @@ grpc_cc_library(
     ],
     deps = [
         "activity",
-        "basic_seq",
         "event_engine_memory_allocator",
         "exec_ctx_wakeup_scheduler",
         "experiments",
diff --git a/src/core/ext/filters/channel_idle/channel_idle_filter.cc b/src/core/ext/filters/channel_idle/channel_idle_filter.cc
index 34601e5e3a0..5a7d98efc47 100644
--- a/src/core/ext/filters/channel_idle/channel_idle_filter.cc
+++ b/src/core/ext/filters/channel_idle/channel_idle_filter.cc
@@ -19,6 +19,7 @@
 
 #include "src/core/ext/filters/channel_idle/channel_idle_filter.h"
 
+#include <stdint.h>
 #include <stdlib.h>
 
 #include <functional>
diff --git a/src/core/lib/channel/connected_channel.cc b/src/core/lib/channel/connected_channel.cc
index 83dcb93845b..29f0924b665 100644
--- a/src/core/lib/channel/connected_channel.cc
+++ b/src/core/lib/channel/connected_channel.cc
@@ -57,7 +57,6 @@
 #include "src/core/lib/promise/activity.h"
 #include "src/core/lib/promise/arena_promise.h"
 #include "src/core/lib/promise/context.h"
-#include "src/core/lib/promise/detail/basic_seq.h"
 #include "src/core/lib/promise/detail/status.h"
 #include "src/core/lib/promise/for_each.h"
 #include "src/core/lib/promise/if.h"
diff --git a/src/core/lib/channel/promise_based_filter.cc b/src/core/lib/channel/promise_based_filter.cc
index 27f3e5bca8d..f81bcc08ca2 100644
--- a/src/core/lib/channel/promise_based_filter.cc
+++ b/src/core/lib/channel/promise_based_filter.cc
@@ -38,7 +38,7 @@
 #include "src/core/lib/gprpp/manual_constructor.h"
 #include "src/core/lib/gprpp/status_helper.h"
 #include "src/core/lib/iomgr/error.h"
-#include "src/core/lib/promise/detail/basic_seq.h"
+#include "src/core/lib/promise/seq.h"
 #include "src/core/lib/slice/slice.h"
 
 extern grpc_core::TraceFlag grpc_trace_channel;
diff --git a/src/core/lib/promise/detail/basic_seq.h b/src/core/lib/promise/detail/basic_seq.h
index 3ebb251a961..663609d4192 100644
--- a/src/core/lib/promise/detail/basic_seq.h
+++ b/src/core/lib/promise/detail/basic_seq.h
@@ -17,384 +17,13 @@
 
 #include <grpc/support/port_platform.h>
 
-#include <array>
-#include <cassert>
-#include <new>
-#include <tuple>
-#include <utility>
-
-#include "absl/meta/type_traits.h"
-#include "absl/utility/utility.h"
-
 #include "src/core/lib/gprpp/construct_destruct.h"
-#include "src/core/lib/promise/detail/promise_factory.h"
-#include "src/core/lib/promise/detail/promise_like.h"
-#include "src/core/lib/promise/detail/switch.h"
 #include "src/core/lib/promise/poll.h"
 
 namespace grpc_core {
 namespace promise_detail {
 
-// Helper for SeqState to evaluate some common types to all partial
-// specializations.
-template <template <typename> class Traits, typename FPromise, typename FNext>
-struct SeqStateTypes {
-  // Our current promise.
-  using Promise = FPromise;
-  // The result of our current promise.
-  using PromiseResult = typename Promise::Result;
-  // Traits around the result of our promise.
-  using PromiseResultTraits = Traits<PromiseResult>;
-  // Wrap the factory callable in our factory wrapper to deal with common edge
-  // cases. We use the 'unwrapped type' from the traits, so for instance, TrySeq
-  // can pass back a T from a StatusOr<T>.
-  using Next = promise_detail::OncePromiseFactory<
-      typename PromiseResultTraits::UnwrappedType, FNext>;
-};
-
-// One state in a sequence.
-// A state contains the current promise, and the promise factory to turn the
-// result of the current promise into the next state's promise. We play a shell
-// game such that the prior state and our current promise are kept in a union,
-// and the next promise factory is kept alongside in the state struct.
-// Recursively this guarantees that the next functions get initialized once, and
-// destroyed once, and don't need to be moved around in between, which avoids a
-// potential O(n**2) loop of next factory moves had we used a variant of states
-// here. The very first state does not have a prior state, and so that state has
-// a partial specialization below. The final state does not have a next state;
-// that state is inlined in BasicSeq since that was simpler to type.
-template <template <typename> class Traits, char I, typename... Fs>
-struct SeqState {
-  // The state evaluated before this state.
-  using PriorState = SeqState<Traits, I - 1, Fs...>;
-  // Initialization from callables.
-  explicit SeqState(std::tuple<Fs*...> fs)
-      : next_factory(std::move(*std::get<I + 1>(fs))) {
-    new (&prior) PriorState(fs);
-  }
-  // Move constructor - assumes we're in the initial state (move prior) as it's
-  // illegal to move a promise after polling it.
-  SeqState(SeqState&& other) noexcept
-      : next_factory(std::move(other.next_factory)) {
-    new (&prior) PriorState(std::move(other.prior));
-  }
-  // Copy constructor - assumes we're in the initial state (move prior) as it's
-  // illegal to move a promise after polling it.
-  SeqState(const SeqState& other) : next_factory(other.next_factory) {
-    new (&prior) PriorState(std::move(other.prior));
-  }
-  // Empty destructor - we instead destruct the innards in BasicSeq manually
-  // depending on state.
-  ~SeqState() {}
-  // Evaluate the current promise, next promise factory types for this state.
-  // The current promise is the next promise from the prior state.
-  // The next factory callable is from the callables passed in:
-  // Fs[0] is the initial promise, Fs[1] is the state 0 next factory, Fs[2] is
-  // the state 1 next factory, etc...
-  using Types = SeqStateTypes<
-      Traits, typename PriorState::Types::Next::Promise,
-      typename std::tuple_element<I + 1, std::tuple<Fs...>>::type>;
-  // Storage for either the current promise or the prior state.
-  union {
-    // If we're in the prior state.
-    GPR_NO_UNIQUE_ADDRESS PriorState prior;
-    // The callables representing our promise.
-    GPR_NO_UNIQUE_ADDRESS typename Types::Promise current_promise;
-  };
-  // Storage for the next promise factory.
-  GPR_NO_UNIQUE_ADDRESS typename Types::Next next_factory;
-};
-
-// Partial specialization of SeqState above for the first state - it has no
-// prior state, so we take the first callable from the template arg list and use
-// it as a promise.
-template <template <typename> class Traits, typename... Fs>
-struct SeqState<Traits, 0, Fs...> {
-  // Initialization from callables.
-  explicit SeqState(std::tuple<Fs*...> args)
-      : current_promise(std::move(*std::get<0>(args))),
-        next_factory(std::move(*std::get<1>(args))) {}
-  // Move constructor - it's assumed we're in this state (see above).
-  SeqState(SeqState&& other) noexcept
-      : current_promise(std::move(other.current_promise)),
-        next_factory(std::move(other.next_factory)) {}
-  // Copy constructor - it's assumed we're in this state (see above).
-  SeqState(const SeqState& other)
-      : current_promise(other.current_promise),
-        next_factory(other.next_factory) {}
-  // Empty destructor - we instead destruct the innards in BasicSeq manually
-  // depending on state.
-  ~SeqState() {}
-  // Evaluate the current promise, next promise factory types for this state.
-  // Our callable is the first element of Fs, wrapped in PromiseLike to handle
-  // some common edge cases. The next factory is the second element.
-  using Types = SeqStateTypes<
-      Traits,
-      PromiseLike<typename std::tuple_element<0, std::tuple<Fs...>>::type>,
-      typename std::tuple_element<1, std::tuple<Fs...>>::type>;
-  GPR_NO_UNIQUE_ADDRESS typename Types::Promise current_promise;
-  GPR_NO_UNIQUE_ADDRESS typename Types::Next next_factory;
-};
-
-// Helper to get a specific state index.
-// Calls the prior state, unless it's this state, in which case it returns
-// that.
-template <char I, template <typename> class Traits, char J, typename... Fs>
-struct GetSeqStateInner {
-  static SeqState<Traits, I, Fs...>* f(SeqState<Traits, J, Fs...>* p) {
-    return GetSeqStateInner<I, Traits, J - 1, Fs...>::f(&p->prior);
-  }
-};
-
-template <char I, template <typename> class Traits, typename... Fs>
-struct GetSeqStateInner<I, Traits, I, Fs...> {
-  static SeqState<Traits, I, Fs...>* f(SeqState<Traits, I, Fs...>* p) {
-    return p;
-  }
-};
-
-template <char I, template <typename> class Traits, char J, typename... Fs>
-absl::enable_if_t<I <= J, SeqState<Traits, I, Fs...>*> GetSeqState(
-    SeqState<Traits, J, Fs...>* p) {
-  return GetSeqStateInner<I, Traits, J, Fs...>::f(p);
-}
-
-template <template <typename> class Traits, char I, typename... Fs, typename T>
-auto CallNext(SeqState<Traits, I, Fs...>* state, T&& arg)
-    -> decltype(SeqState<Traits, I, Fs...>::Types::PromiseResultTraits::
-                    CallFactory(&state->next_factory, std::forward<T>(arg))) {
-  return SeqState<Traits, I, Fs...>::Types::PromiseResultTraits::CallFactory(
-      &state->next_factory, std::forward<T>(arg));
-}
-
-// A sequence under some traits for some set of callables Fs.
-// Fs[0] should be a promise-like object that yields a value.
-// Fs[1..] should be promise-factory-like objects that take the value from the
-// previous step and yield a promise. Note that most of the machinery in
-// PromiseFactory exists to make it possible for those promise-factory-like
-// objects to be anything that's convenient. Traits defines how we move from one
-// step to the next. Traits sets up the wrapping and escape handling for the
-// sequence. Promises return wrapped values that the trait can inspect and
-// unwrap before passing them to the next element of the sequence. The trait can
-// also interpret a wrapped value as an escape value, which terminates
-// evaluation of the sequence immediately yielding a result. Traits for type T
-// have the members:
-//  * type UnwrappedType - the type after removing wrapping from T (i.e. for
-//    TrySeq, T=StatusOr<U> yields UnwrappedType=U).
-//  * type WrappedType - the type after adding wrapping if it doesn't already
-//    exist (i.e. for TrySeq if T is not Status/StatusOr/void, then
-//    WrappedType=StatusOr<T>; if T is Status then WrappedType=Status (it's
-//    already wrapped!))
-//  * template <typename Next> void CallFactory(Next* next_factory, T&& value) -
-//    call promise factory next_factory with the result of unwrapping value, and
-//    return the resulting promise.
-//  * template <typename Result, typename RunNext> Poll<Result>
-//    CheckResultAndRunNext(T prior, RunNext run_next) - examine the value of
-//    prior, and decide to escape or continue. If escaping, return the final
-//    sequence value of type Poll<Result>. If continuing, return the value of
-//    run_next(std::move(prior)).
-template <template <typename> class Traits, typename... Fs>
-class BasicSeq {
- private:
-  // Number of states in the sequence - we'll refer to this some!
-  static constexpr char N = sizeof...(Fs);
-
-  // Current state.
-  static_assert(N < 128, "Long sequence... please revisit BasicSeq");
-  char state_ = 0;
-  // The penultimate state contains all the preceding states too.
-  using PenultimateState = SeqState<Traits, N - 2, Fs...>;
-  // The final state is simply the final promise, which is the next promise from
-  // the penultimate state.
-  using FinalPromise = typename PenultimateState::Types::Next::Promise;
-  union {
-    GPR_NO_UNIQUE_ADDRESS PenultimateState penultimate_state_;
-    GPR_NO_UNIQUE_ADDRESS FinalPromise final_promise_;
-  };
-  using FinalPromiseResult = typename FinalPromise::Result;
-  using Result = typename Traits<FinalPromiseResult>::WrappedType;
-
-  // Get a state by index.
-  template <char I>
-      absl::enable_if_t < I<N - 2, SeqState<Traits, I, Fs...>*> state() {
-    return GetSeqState<I>(&penultimate_state_);
-  }
-
-  template <char I>
-  absl::enable_if_t<I == N - 2, PenultimateState*> state() {
-    return &penultimate_state_;
-  }
-
-  // Get the next state's promise.
-  template <char I>
-  auto next_promise() -> absl::enable_if_t<
-      I != N - 2,
-      decltype(&GetSeqState<I + 1>(static_cast<PenultimateState*>(nullptr))
-                    ->current_promise)> {
-    return &GetSeqState<I + 1>(&penultimate_state_)->current_promise;
-  }
-
-  template <char I>
-  absl::enable_if_t<I == N - 2, FinalPromise*> next_promise() {
-    return &final_promise_;
-  }
-
-  // Callable to advance the state to the next one after I given the result from
-  // state I.
-  template <char I>
-  struct RunNext {
-    BasicSeq* s;
-    template <typename T>
-    Poll<Result> operator()(T&& value) {
-      auto* prior = s->state<I>();
-      using StateType = absl::remove_reference_t<decltype(*prior)>;
-      // Destroy the promise that just completed.
-      Destruct(&prior->current_promise);
-      // Construct the next promise by calling the next promise factory.
-      // We need to ask the traits to do this to deal with value
-      // wrapping/unwrapping.
-      auto n = StateType::Types::PromiseResultTraits::CallFactory(
-          &prior->next_factory, std::forward<T>(value));
-      // Now we also no longer need the factory, so we can destroy that.
-      Destruct(&prior->next_factory);
-      // Constructing the promise for the new state will use the memory
-      // previously occupied by the promise & next factory of the old state.
-      Construct(s->next_promise<I>(), std::move(n));
-      // Store the state counter.
-      s->state_ = I + 1;
-      // Recursively poll the new current state.
-      return s->RunState<I + 1>();
-    }
-  };
-
-  // Poll the current state, advance it if necessary.
-  template <char I>
-  absl::enable_if_t<I != N - 1, Poll<Result>> RunState() {
-    // Get a pointer to the state object.
-    auto* s = state<I>();
-    // Poll the current promise in this state.
-    auto r = s->current_promise();
-    // If we are still pending, say so by returning.
-    if (r.pending()) return Pending();
-    // Current promise is ready, as the traits to do the next thing.
-    // That may be returning - eg if TrySeq sees an error.
-    // Or it may be by calling the callable we hand down - RunNext - which
-    // will advance the state and call the next promise.
-    return Traits<
-        typename absl::remove_reference_t<decltype(*s)>::Types::PromiseResult>::
-        template CheckResultAndRunNext<Result>(std::move(r.value()),
-                                               RunNext<I>{this});
-  }
-
-  // Specialization of RunState to run the final state.
-  template <char I>
-  absl::enable_if_t<I == N - 1, Poll<Result>> RunState() {
-    // Poll the final promise.
-    auto r = final_promise_();
-    // If we are still pending, say so by returning.
-    if (r.pending()) return Pending();
-    // We are complete, return the (wrapped) result.
-    return Result(std::move(r.value()));
-  }
-
-  // For state numbered I, destruct the current promise and the next promise
-  // factory, and recursively destruct the next promise factories for future
-  // states (since they also still exist).
-  template <char I>
-  absl::enable_if_t<I != N - 1, void>
-  DestructCurrentPromiseAndSubsequentFactories() {
-    Destruct(&GetSeqState<I>(&penultimate_state_)->current_promise);
-    DestructSubsequentFactories<I>();
-  }
-
-  template <char I>
-  absl::enable_if_t<I == N - 1, void>
-  DestructCurrentPromiseAndSubsequentFactories() {
-    Destruct(&final_promise_);
-  }
-
-  // For state I, destruct the next promise factory, and recursively the next
-  // promise factories after.
-  template <char I>
-  absl::enable_if_t<I != N - 1, void> DestructSubsequentFactories() {
-    Destruct(&GetSeqState<I>(&penultimate_state_)->next_factory);
-    DestructSubsequentFactories<I + 1>();
-  }
-
-  template <char I>
-  absl::enable_if_t<I == N - 1, void> DestructSubsequentFactories() {}
-
-  // Placate older compilers by wrapping RunState in a struct so that their
-  // parameter unpacking can work.
-  template <char I>
-  struct RunStateStruct {
-    BasicSeq* s;
-    Poll<Result> operator()() { return s->RunState<I>(); }
-  };
-
-  // Similarly placate those compilers for
-  // DestructCurrentPromiseAndSubsequentFactories
-  template <char I>
-  struct DestructCurrentPromiseAndSubsequentFactoriesStruct {
-    BasicSeq* s;
-    void operator()() {
-      return s->DestructCurrentPromiseAndSubsequentFactories<I>();
-    }
-  };
-
-  // Run the current state (and possibly later states if that one finishes).
-  // Single argument is a type that encodes the integer sequence 0, 1, 2, ...,
-  // N-1 as a type, but which uses no memory. This is used to expand out
-  // RunState instances using a template unpack to pass to Switch, which encodes
-  // a switch statement over the various cases. This ultimately gives us a
-  // Duff's device like mechanic for evaluating sequences.
-  template <char... I>
-  Poll<Result> Run(absl::integer_sequence<char, I...>) {
-    return Switch<Poll<Result>>(state_, RunStateStruct<I>{this}...);
-  }
-
-  // Run the appropriate destructors for a given state.
-  // Single argument is a type that encodes the integer sequence 0, 1, 2, ...,
-  // N-1 as a type, but which uses no memory. This is used to expand out
-  // DestructCurrentPromiseAndSubsequentFactories instances to pass to Switch,
-  // which can choose the correct instance at runtime to destroy everything.
-  template <char... I>
-  void RunDestruct(absl::integer_sequence<char, I...>) {
-    Switch<void>(
-        state_, DestructCurrentPromiseAndSubsequentFactoriesStruct<I>{this}...);
-  }
-
- public:
-  // Construct a sequence given the callables that will control it.
-  explicit BasicSeq(Fs... fs) : penultimate_state_(std::make_tuple(&fs...)) {}
-  // No assignment... we don't need it (but if we ever find a good case, then
-  // it's ok to implement).
-  BasicSeq& operator=(const BasicSeq&) = delete;
-  // Copy construction - only for state 0.
-  // It's illegal to copy a Promise after polling it - if we are in state>0 we
-  // *must* have been polled.
-  BasicSeq(const BasicSeq& other) {
-    assert(other.state_ == 0);
-    new (&penultimate_state_) PenultimateState(other.penultimate_state_);
-  }
-  // Move construction - only for state 0.
-  // It's illegal to copy a Promise after polling it - if we are in state>0 we
-  // *must* have been polled.
-  BasicSeq(BasicSeq&& other) noexcept {
-    assert(other.state_ == 0);
-    new (&penultimate_state_)
-        PenultimateState(std::move(other.penultimate_state_));
-  }
-  // Destruct based on current state.
-  ~BasicSeq() { RunDestruct(absl::make_integer_sequence<char, N>()); }
-
-  // Poll the sequence once.
-  Poll<Result> operator()() {
-    return Run(absl::make_integer_sequence<char, N>());
-  }
-};
-
-// As above, but models a sequence of unknown size
+// Models a sequence of unknown size
 // At each element, the accumulator A and the current value V is passed to some
 // function of type IterTraits::Factory as f(V, IterTraits::Argument); f is
 // expected to return a promise that resolves to IterTraits::Wrapped.
diff --git a/src/core/lib/promise/detail/seq_state.h b/src/core/lib/promise/detail/seq_state.h
new file mode 100644
index 00000000000..c5be81b11f0
--- /dev/null
+++ b/src/core/lib/promise/detail/seq_state.h
@@ -0,0 +1,2076 @@
+// Copyright 2023 gRPC authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef GRPC_SRC_CORE_LIB_PROMISE_DETAIL_SEQ_STATE_H
+#define GRPC_SRC_CORE_LIB_PROMISE_DETAIL_SEQ_STATE_H
+
+// This file is generated by tools/codegen/core/gen_seq.py
+
+#include <grpc/support/port_platform.h>
+
+#include <stdint.h>
+
+#include <utility>
+
+#include "absl/base/attributes.h"
+
+#include <grpc/support/log.h>
+
+#include "src/core/lib/gprpp/construct_destruct.h"
+#include "src/core/lib/promise/detail/promise_factory.h"
+#include "src/core/lib/promise/detail/promise_like.h"
+#include "src/core/lib/promise/poll.h"
+
+// A sequence under some traits for some set of callables P, Fs.
+// P should be a promise-like object that yields a value.
+// Fs... should be promise-factory-like objects that take the value from the
+// previous step and yield a promise. Note that most of the machinery in
+// PromiseFactory exists to make it possible for those promise-factory-like
+// objects to be anything that's convenient.
+// Traits defines how we move from one step to the next. Traits sets up the
+// wrapping and escape handling for the sequence.
+// Promises return wrapped values that the trait can inspect and unwrap before
+// passing them to the next element of the sequence. The trait can
+// also interpret a wrapped value as an escape value, which terminates
+// evaluation of the sequence immediately yielding a result. Traits for type T
+// have the members:
+//  * type UnwrappedType - the type after removing wrapping from T (i.e. for
+//    TrySeq, T=StatusOr<U> yields UnwrappedType=U).
+//  * type WrappedType - the type after adding wrapping if it doesn't already
+//    exist (i.e. for TrySeq if T is not Status/StatusOr/void, then
+//    WrappedType=StatusOr<T>; if T is Status then WrappedType=Status (it's
+//    already wrapped!))
+//  * template <typename Next> void CallFactory(Next* next_factory, T&& value) -
+//    call promise factory next_factory with the result of unwrapping value, and
+//    return the resulting promise.
+//  * template <typename Result, typename RunNext> Poll<Result>
+//    CheckResultAndRunNext(T prior, RunNext run_next) - examine the value of
+//    prior, and decide to escape or continue. If escaping, return the final
+//    sequence value of type Poll<Result>. If continuing, return the value of
+//    run_next(std::move(prior)).
+//
+// A state contains the current promise, and the promise factory to turn the
+// result of the current promise into the next state's promise. We play a shell
+// game such that the prior state and our current promise are kept in a union,
+// and the next promise factory is kept alongside in the state struct.
+// Recursively this guarantees that the next functions get initialized once, and
+// destroyed once, and don't need to be moved around in between, which avoids a
+// potential O(n**2) loop of next factory moves had we used a variant of states
+// here. The very first state does not have a prior state, and so that state has
+// a partial specialization below. The final state does not have a next state;
+// that state is inlined in BasicSeq since that was simpler to type.
+
+namespace grpc_core {
+namespace promise_detail {
+template <template <typename> class Traits, typename P, typename... Fs>
+struct SeqState;
+
+template <template <typename> class Traits, typename P, typename F0>
+struct SeqState<Traits, P, F0> {
+  using Promise0 = PromiseLike<P>;
+  using PromiseResult0 = typename Promise0::Result;
+  using PromiseResultTraits0 = Traits<PromiseResult0>;
+  using NextFactory0 =
+      OncePromiseFactory<typename PromiseResultTraits0::UnwrappedType, F0>;
+  using Promise1 = typename NextFactory0::Promise;
+  using PromiseResult1 = typename Promise1::Result;
+  using PromiseResultTraits1 = Traits<PromiseResult1>;
+  using Result = typename PromiseResultTraits1::WrappedType;
+  struct Running0 {
+    GPR_NO_UNIQUE_ADDRESS Promise0 current_promise;
+    GPR_NO_UNIQUE_ADDRESS NextFactory0 next_factory;
+  };
+  union {
+    GPR_NO_UNIQUE_ADDRESS Running0 prior;
+    GPR_NO_UNIQUE_ADDRESS Promise1 current_promise;
+  };
+  enum class State : uint8_t { kState0, kState1 };
+  GPR_NO_UNIQUE_ADDRESS State state = State::kState0;
+
+  SeqState(P&& p, F0&& f0) noexcept {
+    Construct(&prior.current_promise, std::forward<P>(p));
+    Construct(&prior.next_factory, std::forward<F0>(f0));
+  }
+  ~SeqState() {
+    switch (state) {
+      case State::kState0:
+        Destruct(&prior.current_promise);
+        goto tail0;
+      case State::kState1:
+        Destruct(&current_promise);
+        return;
+    }
+  tail0:
+    Destruct(&prior.next_factory);
+  }
+  SeqState(const SeqState& other) noexcept : state(other.state) {
+    GPR_ASSERT(state == State::kState0);
+    Construct(&prior.current_promise, other.prior.current_promise);
+    Construct(&prior.next_factory, other.prior.next_factory);
+  }
+  SeqState& operator=(const SeqState& other) = delete;
+  SeqState(SeqState&& other) noexcept : state(other.state) {
+    switch (state) {
+      case State::kState0:
+        Construct(&prior.current_promise,
+                  std::move(other.prior.current_promise));
+        goto tail0;
+      case State::kState1:
+        Construct(&current_promise, std::move(other.current_promise));
+        return;
+    }
+  tail0:
+    Construct(&prior.next_factory, std::move(other.prior.next_factory));
+  }
+  SeqState& operator=(SeqState&& other) = delete;
+  Poll<Result> PollOnce() {
+    switch (state) {
+      case State::kState0: {
+        auto result = prior.current_promise();
+        PromiseResult0* p = result.value_if_ready();
+        if (p == nullptr) return Pending{};
+        if (!PromiseResultTraits0::IsOk(*p)) {
+          return PromiseResultTraits0::template ReturnValue<Result>(
+              std::move(*p));
+        }
+        Destruct(&prior.current_promise);
+        auto next_promise = PromiseResultTraits0::CallFactory(
+            &prior.next_factory, std::move(*p));
+        Destruct(&prior.next_factory);
+        Construct(&current_promise, std::move(next_promise));
+        state = State::kState1;
+      }
+        ABSL_FALLTHROUGH_INTENDED;
+      default:
+      case State::kState1: {
+        auto result = current_promise();
+        auto* p = result.value_if_ready();
+        if (p == nullptr) return Pending{};
+        return Result(std::move(*p));
+      }
+    }
+  }
+};
+
+template <template <typename> class Traits, typename P, typename F0,
+          typename F1>
+struct SeqState<Traits, P, F0, F1> {
+  using Promise0 = PromiseLike<P>;
+  using PromiseResult0 = typename Promise0::Result;
+  using PromiseResultTraits0 = Traits<PromiseResult0>;
+  using NextFactory0 =
+      OncePromiseFactory<typename PromiseResultTraits0::UnwrappedType, F0>;
+  using Promise1 = typename NextFactory0::Promise;
+  using PromiseResult1 = typename Promise1::Result;
+  using PromiseResultTraits1 = Traits<PromiseResult1>;
+  using NextFactory1 =
+      OncePromiseFactory<typename PromiseResultTraits1::UnwrappedType, F1>;
+  using Promise2 = typename NextFactory1::Promise;
+  using PromiseResult2 = typename Promise2::Result;
+  using PromiseResultTraits2 = Traits<PromiseResult2>;
+  using Result = typename PromiseResultTraits2::WrappedType;
+  struct Running0 {
+    GPR_NO_UNIQUE_ADDRESS Promise0 current_promise;
+    GPR_NO_UNIQUE_ADDRESS NextFactory0 next_factory;
+  };
+  struct Running1 {
+    union {
+      GPR_NO_UNIQUE_ADDRESS Running0 prior;
+      GPR_NO_UNIQUE_ADDRESS Promise1 current_promise;
+    };
+    GPR_NO_UNIQUE_ADDRESS NextFactory1 next_factory;
+  };
+  union {
+    GPR_NO_UNIQUE_ADDRESS Running1 prior;
+    GPR_NO_UNIQUE_ADDRESS Promise2 current_promise;
+  };
+  enum class State : uint8_t { kState0, kState1, kState2 };
+  GPR_NO_UNIQUE_ADDRESS State state = State::kState0;
+
+  SeqState(P&& p, F0&& f0, F1&& f1) noexcept {
+    Construct(&prior.prior.current_promise, std::forward<P>(p));
+    Construct(&prior.prior.next_factory, std::forward<F0>(f0));
+    Construct(&prior.next_factory, std::forward<F1>(f1));
+  }
+  ~SeqState() {
+    switch (state) {
+      case State::kState0:
+        Destruct(&prior.prior.current_promise);
+        goto tail0;
+      case State::kState1:
+        Destruct(&prior.current_promise);
+        goto tail1;
+      case State::kState2:
+        Destruct(&current_promise);
+        return;
+    }
+  tail0:
+    Destruct(&prior.prior.next_factory);
+  tail1:
+    Destruct(&prior.next_factory);
+  }
+  SeqState(const SeqState& other) noexcept : state(other.state) {
+    GPR_ASSERT(state == State::kState0);
+    Construct(&prior.current_promise, other.prior.current_promise);
+    Construct(&prior.prior.next_factory, other.prior.prior.next_factory);
+    Construct(&prior.next_factory, other.prior.next_factory);
+  }
+  SeqState& operator=(const SeqState& other) = delete;
+  SeqState(SeqState&& other) noexcept : state(other.state) {
+    switch (state) {
+      case State::kState0:
+        Construct(&prior.prior.current_promise,
+                  std::move(other.prior.prior.current_promise));
+        goto tail0;
+      case State::kState1:
+        Construct(&prior.current_promise,
+                  std::move(other.prior.current_promise));
+        goto tail1;
+      case State::kState2:
+        Construct(&current_promise, std::move(other.current_promise));
+        return;
+    }
+  tail0:
+    Construct(&prior.prior.next_factory,
+              std::move(other.prior.prior.next_factory));
+  tail1:
+    Construct(&prior.next_factory, std::move(other.prior.next_factory));
+  }
+  SeqState& operator=(SeqState&& other) = delete;
+  Poll<Result> PollOnce() {
+    switch (state) {
+      case State::kState0: {
+        auto result = prior.prior.current_promise();
+        PromiseResult0* p = result.value_if_ready();
+        if (p == nullptr) return Pending{};
+        if (!PromiseResultTraits0::IsOk(*p)) {
+          return PromiseResultTraits0::template ReturnValue<Result>(
+              std::move(*p));
+        }
+        Destruct(&prior.prior.current_promise);
+        auto next_promise = PromiseResultTraits0::CallFactory(
+            &prior.prior.next_factory, std::move(*p));
+        Destruct(&prior.prior.next_factory);
+        Construct(&prior.current_promise, std::move(next_promise));
+        state = State::kState1;
+      }
+        ABSL_FALLTHROUGH_INTENDED;
+      case State::kState1: {
+        auto result = prior.current_promise();
+        PromiseResult1* p = result.value_if_ready();
+        if (p == nullptr) return Pending{};
+        if (!PromiseResultTraits1::IsOk(*p)) {
+          return PromiseResultTraits1::template ReturnValue<Result>(
+              std::move(*p));
+        }
+        Destruct(&prior.current_promise);
+        auto next_promise = PromiseResultTraits1::CallFactory(
+            &prior.next_factory, std::move(*p));
+        Destruct(&prior.next_factory);
+        Construct(&current_promise, std::move(next_promise));
+        state = State::kState2;
+      }
+        ABSL_FALLTHROUGH_INTENDED;
+      default:
+      case State::kState2: {
+        auto result = current_promise();
+        auto* p = result.value_if_ready();
+        if (p == nullptr) return Pending{};
+        return Result(std::move(*p));
+      }
+    }
+  }
+};
+
+template <template <typename> class Traits, typename P, typename F0,
+          typename F1, typename F2>
+struct SeqState<Traits, P, F0, F1, F2> {
+  using Promise0 = PromiseLike<P>;
+  using PromiseResult0 = typename Promise0::Result;
+  using PromiseResultTraits0 = Traits<PromiseResult0>;
+  using NextFactory0 =
+      OncePromiseFactory<typename PromiseResultTraits0::UnwrappedType, F0>;
+  using Promise1 = typename NextFactory0::Promise;
+  using PromiseResult1 = typename Promise1::Result;
+  using PromiseResultTraits1 = Traits<PromiseResult1>;
+  using NextFactory1 =
+      OncePromiseFactory<typename PromiseResultTraits1::UnwrappedType, F1>;
+  using Promise2 = typename NextFactory1::Promise;
+  using PromiseResult2 = typename Promise2::Result;
+  using PromiseResultTraits2 = Traits<PromiseResult2>;
+  using NextFactory2 =
+      OncePromiseFactory<typename PromiseResultTraits2::UnwrappedType, F2>;
+  using Promise3 = typename NextFactory2::Promise;
+  using PromiseResult3 = typename Promise3::Result;
+  using PromiseResultTraits3 = Traits<PromiseResult3>;
+  using Result = typename PromiseResultTraits3::WrappedType;
+  struct Running0 {
+    GPR_NO_UNIQUE_ADDRESS Promise0 current_promise;
+    GPR_NO_UNIQUE_ADDRESS NextFactory0 next_factory;
+  };
+  struct Running1 {
+    union {
+      GPR_NO_UNIQUE_ADDRESS Running0 prior;
+      GPR_NO_UNIQUE_ADDRESS Promise1 current_promise;
+    };
+    GPR_NO_UNIQUE_ADDRESS NextFactory1 next_factory;
+  };
+  struct Running2 {
+    union {
+      GPR_NO_UNIQUE_ADDRESS Running1 prior;
+      GPR_NO_UNIQUE_ADDRESS Promise2 current_promise;
+    };
+    GPR_NO_UNIQUE_ADDRESS NextFactory2 next_factory;
+  };
+  union {
+    GPR_NO_UNIQUE_ADDRESS Running2 prior;
+    GPR_NO_UNIQUE_ADDRESS Promise3 current_promise;
+  };
+  enum class State : uint8_t { kState0, kState1, kState2, kState3 };
+  GPR_NO_UNIQUE_ADDRESS State state = State::kState0;
+
+  SeqState(P&& p, F0&& f0, F1&& f1, F2&& f2) noexcept {
+    Construct(&prior.prior.prior.current_promise, std::forward<P>(p));
+    Construct(&prior.prior.prior.next_factory, std::forward<F0>(f0));
+    Construct(&prior.prior.next_factory, std::forward<F1>(f1));
+    Construct(&prior.next_factory, std::forward<F2>(f2));
+  }
+  ~SeqState() {
+    switch (state) {
+      case State::kState0:
+        Destruct(&prior.prior.prior.current_promise);
+        goto tail0;
+      case State::kState1:
+        Destruct(&prior.prior.current_promise);
+        goto tail1;
+      case State::kState2:
+        Destruct(&prior.current_promise);
+        goto tail2;
+      case State::kState3:
+        Destruct(&current_promise);
+        return;
+    }
+  tail0:
+    Destruct(&prior.prior.prior.next_factory);
+  tail1:
+    Destruct(&prior.prior.next_factory);
+  tail2:
+    Destruct(&prior.next_factory);
+  }
+  SeqState(const SeqState& other) noexcept : state(other.state) {
+    GPR_ASSERT(state == State::kState0);
+    Construct(&prior.current_promise, other.prior.current_promise);
+    Construct(&prior.prior.prior.next_factory,
+              other.prior.prior.prior.next_factory);
+    Construct(&prior.prior.next_factory, other.prior.prior.next_factory);
+    Construct(&prior.next_factory, other.prior.next_factory);
+  }
+  SeqState& operator=(const SeqState& other) = delete;
+  SeqState(SeqState&& other) noexcept : state(other.state) {
+    switch (state) {
+      case State::kState0:
+        Construct(&prior.prior.prior.current_promise,
+                  std::move(other.prior.prior.prior.current_promise));
+        goto tail0;
+      case State::kState1:
+        Construct(&prior.prior.current_promise,
+                  std::move(other.prior.prior.current_promise));
+        goto tail1;
+      case State::kState2:
+        Construct(&prior.current_promise,
+                  std::move(other.prior.current_promise));
+        goto tail2;
+      case State::kState3:
+        Construct(&current_promise, std::move(other.current_promise));
+        return;
+    }
+  tail0:
+    Construct(&prior.prior.prior.next_factory,
+              std::move(other.prior.prior.prior.next_factory));
+  tail1:
+    Construct(&prior.prior.next_factory,
+              std::move(other.prior.prior.next_factory));
+  tail2:
+    Construct(&prior.next_factory, std::move(other.prior.next_factory));
+  }
+  SeqState& operator=(SeqState&& other) = delete;
+  Poll<Result> PollOnce() {
+    switch (state) {
+      case State::kState0: {
+        auto result = prior.prior.prior.current_promise();
+        PromiseResult0* p = result.value_if_ready();
+        if (p == nullptr) return Pending{};
+        if (!PromiseResultTraits0::IsOk(*p)) {
+          return PromiseResultTraits0::template ReturnValue<Result>(
+              std::move(*p));
+        }
+        Destruct(&prior.prior.prior.current_promise);
+        auto next_promise = PromiseResultTraits0::CallFactory(
+            &prior.prior.prior.next_factory, std::move(*p));
+        Destruct(&prior.prior.prior.next_factory);
+        Construct(&prior.prior.current_promise, std::move(next_promise));
+        state = State::kState1;
+      }
+        ABSL_FALLTHROUGH_INTENDED;
+      case State::kState1: {
+        auto result = prior.prior.current_promise();
+        PromiseResult1* p = result.value_if_ready();
+        if (p == nullptr) return Pending{};
+        if (!PromiseResultTraits1::IsOk(*p)) {
+          return PromiseResultTraits1::template ReturnValue<Result>(
+              std::move(*p));
+        }
+        Destruct(&prior.prior.current_promise);
+        auto next_promise = PromiseResultTraits1::CallFactory(
+            &prior.prior.next_factory, std::move(*p));
+        Destruct(&prior.prior.next_factory);
+        Construct(&prior.current_promise, std::move(next_promise));
+        state = State::kState2;
+      }
+        ABSL_FALLTHROUGH_INTENDED;
+      case State::kState2: {
+        auto result = prior.current_promise();
+        PromiseResult2* p = result.value_if_ready();
+        if (p == nullptr) return Pending{};
+        if (!PromiseResultTraits2::IsOk(*p)) {
+          return PromiseResultTraits2::template ReturnValue<Result>(
+              std::move(*p));
+        }
+        Destruct(&prior.current_promise);
+        auto next_promise = PromiseResultTraits2::CallFactory(
+            &prior.next_factory, std::move(*p));
+        Destruct(&prior.next_factory);
+        Construct(&current_promise, std::move(next_promise));
+        state = State::kState3;
+      }
+        ABSL_FALLTHROUGH_INTENDED;
+      default:
+      case State::kState3: {
+        auto result = current_promise();
+        auto* p = result.value_if_ready();
+        if (p == nullptr) return Pending{};
+        return Result(std::move(*p));
+      }
+    }
+  }
+};
+
+template <template <typename> class Traits, typename P, typename F0,
+          typename F1, typename F2, typename F3>
+struct SeqState<Traits, P, F0, F1, F2, F3> {
+  using Promise0 = PromiseLike<P>;
+  using PromiseResult0 = typename Promise0::Result;
+  using PromiseResultTraits0 = Traits<PromiseResult0>;
+  using NextFactory0 =
+      OncePromiseFactory<typename PromiseResultTraits0::UnwrappedType, F0>;
+  using Promise1 = typename NextFactory0::Promise;
+  using PromiseResult1 = typename Promise1::Result;
+  using PromiseResultTraits1 = Traits<PromiseResult1>;
+  using NextFactory1 =
+      OncePromiseFactory<typename PromiseResultTraits1::UnwrappedType, F1>;
+  using Promise2 = typename NextFactory1::Promise;
+  using PromiseResult2 = typename Promise2::Result;
+  using PromiseResultTraits2 = Traits<PromiseResult2>;
+  using NextFactory2 =
+      OncePromiseFactory<typename PromiseResultTraits2::UnwrappedType, F2>;
+  using Promise3 = typename NextFactory2::Promise;
+  using PromiseResult3 = typename Promise3::Result;
+  using PromiseResultTraits3 = Traits<PromiseResult3>;
+  using NextFactory3 =
+      OncePromiseFactory<typename PromiseResultTraits3::UnwrappedType, F3>;
+  using Promise4 = typename NextFactory3::Promise;
+  using PromiseResult4 = typename Promise4::Result;
+  using PromiseResultTraits4 = Traits<PromiseResult4>;
+  using Result = typename PromiseResultTraits4::WrappedType;
+  struct Running0 {
+    GPR_NO_UNIQUE_ADDRESS Promise0 current_promise;
+    GPR_NO_UNIQUE_ADDRESS NextFactory0 next_factory;
+  };
+  struct Running1 {
+    union {
+      GPR_NO_UNIQUE_ADDRESS Running0 prior;
+      GPR_NO_UNIQUE_ADDRESS Promise1 current_promise;
+    };
+    GPR_NO_UNIQUE_ADDRESS NextFactory1 next_factory;
+  };
+  struct Running2 {
+    union {
+      GPR_NO_UNIQUE_ADDRESS Running1 prior;
+      GPR_NO_UNIQUE_ADDRESS Promise2 current_promise;
+    };
+    GPR_NO_UNIQUE_ADDRESS NextFactory2 next_factory;
+  };
+  struct Running3 {
+    union {
+      GPR_NO_UNIQUE_ADDRESS Running2 prior;
+      GPR_NO_UNIQUE_ADDRESS Promise3 current_promise;
+    };
+    GPR_NO_UNIQUE_ADDRESS NextFactory3 next_factory;
+  };
+  union {
+    GPR_NO_UNIQUE_ADDRESS Running3 prior;
+    GPR_NO_UNIQUE_ADDRESS Promise4 current_promise;
+  };
+  enum class State : uint8_t { kState0, kState1, kState2, kState3, kState4 };
+  GPR_NO_UNIQUE_ADDRESS State state = State::kState0;
+
+  SeqState(P&& p, F0&& f0, F1&& f1, F2&& f2, F3&& f3) noexcept {
+    Construct(&prior.prior.prior.prior.current_promise, std::forward<P>(p));
+    Construct(&prior.prior.prior.prior.next_factory, std::forward<F0>(f0));
+    Construct(&prior.prior.prior.next_factory, std::forward<F1>(f1));
+    Construct(&prior.prior.next_factory, std::forward<F2>(f2));
+    Construct(&prior.next_factory, std::forward<F3>(f3));
+  }
+  ~SeqState() {
+    switch (state) {
+      case State::kState0:
+        Destruct(&prior.prior.prior.prior.current_promise);
+        goto tail0;
+      case State::kState1:
+        Destruct(&prior.prior.prior.current_promise);
+        goto tail1;
+      case State::kState2:
+        Destruct(&prior.prior.current_promise);
+        goto tail2;
+      case State::kState3:
+        Destruct(&prior.current_promise);
+        goto tail3;
+      case State::kState4:
+        Destruct(&current_promise);
+        return;
+    }
+  tail0:
+    Destruct(&prior.prior.prior.prior.next_factory);
+  tail1:
+    Destruct(&prior.prior.prior.next_factory);
+  tail2:
+    Destruct(&prior.prior.next_factory);
+  tail3:
+    Destruct(&prior.next_factory);
+  }
+  SeqState(const SeqState& other) noexcept : state(other.state) {
+    GPR_ASSERT(state == State::kState0);
+    Construct(&prior.current_promise, other.prior.current_promise);
+    Construct(&prior.prior.prior.prior.next_factory,
+              other.prior.prior.prior.prior.next_factory);
+    Construct(&prior.prior.prior.next_factory,
+              other.prior.prior.prior.next_factory);
+    Construct(&prior.prior.next_factory, other.prior.prior.next_factory);
+    Construct(&prior.next_factory, other.prior.next_factory);
+  }
+  SeqState& operator=(const SeqState& other) = delete;
+  SeqState(SeqState&& other) noexcept : state(other.state) {
+    switch (state) {
+      case State::kState0:
+        Construct(&prior.prior.prior.prior.current_promise,
+                  std::move(other.prior.prior.prior.prior.current_promise));
+        goto tail0;
+      case State::kState1:
+        Construct(&prior.prior.prior.current_promise,
+                  std::move(other.prior.prior.prior.current_promise));
+        goto tail1;
+      case State::kState2:
+        Construct(&prior.prior.current_promise,
+                  std::move(other.prior.prior.current_promise));
+        goto tail2;
+      case State::kState3:
+        Construct(&prior.current_promise,
+                  std::move(other.prior.current_promise));
+        goto tail3;
+      case State::kState4:
+        Construct(&current_promise, std::move(other.current_promise));
+        return;
+    }
+  tail0:
+    Construct(&prior.prior.prior.prior.next_factory,
+              std::move(other.prior.prior.prior.prior.next_factory));
+  tail1:
+    Construct(&prior.prior.prior.next_factory,
+              std::move(other.prior.prior.prior.next_factory));
+  tail2:
+    Construct(&prior.prior.next_factory,
+              std::move(other.prior.prior.next_factory));
+  tail3:
+    Construct(&prior.next_factory, std::move(other.prior.next_factory));
+  }
+  SeqState& operator=(SeqState&& other) = delete;
+  Poll<Result> PollOnce() {
+    switch (state) {
+      case State::kState0: {
+        auto result = prior.prior.prior.prior.current_promise();
+        PromiseResult0* p = result.value_if_ready();
+        if (p == nullptr) return Pending{};
+        if (!PromiseResultTraits0::IsOk(*p)) {
+          return PromiseResultTraits0::template ReturnValue<Result>(
+              std::move(*p));
+        }
+        Destruct(&prior.prior.prior.prior.current_promise);
+        auto next_promise = PromiseResultTraits0::CallFactory(
+            &prior.prior.prior.prior.next_factory, std::move(*p));
+        Destruct(&prior.prior.prior.prior.next_factory);
+        Construct(&prior.prior.prior.current_promise, std::move(next_promise));
+        state = State::kState1;
+      }
+        ABSL_FALLTHROUGH_INTENDED;
+      case State::kState1: {
+        auto result = prior.prior.prior.current_promise();
+        PromiseResult1* p = result.value_if_ready();
+        if (p == nullptr) return Pending{};
+        if (!PromiseResultTraits1::IsOk(*p)) {
+          return PromiseResultTraits1::template ReturnValue<Result>(
+              std::move(*p));
+        }
+        Destruct(&prior.prior.prior.current_promise);
+        auto next_promise = PromiseResultTraits1::CallFactory(
+            &prior.prior.prior.next_factory, std::move(*p));
+        Destruct(&prior.prior.prior.next_factory);
+        Construct(&prior.prior.current_promise, std::move(next_promise));
+        state = State::kState2;
+      }
+        ABSL_FALLTHROUGH_INTENDED;
+      case State::kState2: {
+        auto result = prior.prior.current_promise();
+        PromiseResult2* p = result.value_if_ready();
+        if (p == nullptr) return Pending{};
+        if (!PromiseResultTraits2::IsOk(*p)) {
+          return PromiseResultTraits2::template ReturnValue<Result>(
+              std::move(*p));
+        }
+        Destruct(&prior.prior.current_promise);
+        auto next_promise = PromiseResultTraits2::CallFactory(
+            &prior.prior.next_factory, std::move(*p));
+        Destruct(&prior.prior.next_factory);
+        Construct(&prior.current_promise, std::move(next_promise));
+        state = State::kState3;
+      }
+        ABSL_FALLTHROUGH_INTENDED;
+      case State::kState3: {
+        auto result = prior.current_promise();
+        PromiseResult3* p = result.value_if_ready();
+        if (p == nullptr) return Pending{};
+        if (!PromiseResultTraits3::IsOk(*p)) {
+          return PromiseResultTraits3::template ReturnValue<Result>(
+              std::move(*p));
+        }
+        Destruct(&prior.current_promise);
+        auto next_promise = PromiseResultTraits3::CallFactory(
+            &prior.next_factory, std::move(*p));
+        Destruct(&prior.next_factory);
+        Construct(&current_promise, std::move(next_promise));
+        state = State::kState4;
+      }
+        ABSL_FALLTHROUGH_INTENDED;
+      default:
+      case State::kState4: {
+        auto result = current_promise();
+        auto* p = result.value_if_ready();
+        if (p == nullptr) return Pending{};
+        return Result(std::move(*p));
+      }
+    }
+  }
+};
+
+template <template <typename> class Traits, typename P, typename F0,
+          typename F1, typename F2, typename F3, typename F4>
+struct SeqState<Traits, P, F0, F1, F2, F3, F4> {
+  using Promise0 = PromiseLike<P>;
+  using PromiseResult0 = typename Promise0::Result;
+  using PromiseResultTraits0 = Traits<PromiseResult0>;
+  using NextFactory0 =
+      OncePromiseFactory<typename PromiseResultTraits0::UnwrappedType, F0>;
+  using Promise1 = typename NextFactory0::Promise;
+  using PromiseResult1 = typename Promise1::Result;
+  using PromiseResultTraits1 = Traits<PromiseResult1>;
+  using NextFactory1 =
+      OncePromiseFactory<typename PromiseResultTraits1::UnwrappedType, F1>;
+  using Promise2 = typename NextFactory1::Promise;
+  using PromiseResult2 = typename Promise2::Result;
+  using PromiseResultTraits2 = Traits<PromiseResult2>;
+  using NextFactory2 =
+      OncePromiseFactory<typename PromiseResultTraits2::UnwrappedType, F2>;
+  using Promise3 = typename NextFactory2::Promise;
+  using PromiseResult3 = typename Promise3::Result;
+  using PromiseResultTraits3 = Traits<PromiseResult3>;
+  using NextFactory3 =
+      OncePromiseFactory<typename PromiseResultTraits3::UnwrappedType, F3>;
+  using Promise4 = typename NextFactory3::Promise;
+  using PromiseResult4 = typename Promise4::Result;
+  using PromiseResultTraits4 = Traits<PromiseResult4>;
+  using NextFactory4 =
+      OncePromiseFactory<typename PromiseResultTraits4::UnwrappedType, F4>;
+  using Promise5 = typename NextFactory4::Promise;
+  using PromiseResult5 = typename Promise5::Result;
+  using PromiseResultTraits5 = Traits<PromiseResult5>;
+  using Result = typename PromiseResultTraits5::WrappedType;
+  struct Running0 {
+    GPR_NO_UNIQUE_ADDRESS Promise0 current_promise;
+    GPR_NO_UNIQUE_ADDRESS NextFactory0 next_factory;
+  };
+  struct Running1 {
+    union {
+      GPR_NO_UNIQUE_ADDRESS Running0 prior;
+      GPR_NO_UNIQUE_ADDRESS Promise1 current_promise;
+    };
+    GPR_NO_UNIQUE_ADDRESS NextFactory1 next_factory;
+  };
+  struct Running2 {
+    union {
+      GPR_NO_UNIQUE_ADDRESS Running1 prior;
+      GPR_NO_UNIQUE_ADDRESS Promise2 current_promise;
+    };
+    GPR_NO_UNIQUE_ADDRESS NextFactory2 next_factory;
+  };
+  struct Running3 {
+    union {
+      GPR_NO_UNIQUE_ADDRESS Running2 prior;
+      GPR_NO_UNIQUE_ADDRESS Promise3 current_promise;
+    };
+    GPR_NO_UNIQUE_ADDRESS NextFactory3 next_factory;
+  };
+  struct Running4 {
+    union {
+      GPR_NO_UNIQUE_ADDRESS Running3 prior;
+      GPR_NO_UNIQUE_ADDRESS Promise4 current_promise;
+    };
+    GPR_NO_UNIQUE_ADDRESS NextFactory4 next_factory;
+  };
+  union {
+    GPR_NO_UNIQUE_ADDRESS Running4 prior;
+    GPR_NO_UNIQUE_ADDRESS Promise5 current_promise;
+  };
+  enum class State : uint8_t {
+    kState0,
+    kState1,
+    kState2,
+    kState3,
+    kState4,
+    kState5
+  };
+  GPR_NO_UNIQUE_ADDRESS State state = State::kState0;
+
+  SeqState(P&& p, F0&& f0, F1&& f1, F2&& f2, F3&& f3, F4&& f4) noexcept {
+    Construct(&prior.prior.prior.prior.prior.current_promise,
+              std::forward<P>(p));
+    Construct(&prior.prior.prior.prior.prior.next_factory,
+              std::forward<F0>(f0));
+    Construct(&prior.prior.prior.prior.next_factory, std::forward<F1>(f1));
+    Construct(&prior.prior.prior.next_factory, std::forward<F2>(f2));
+    Construct(&prior.prior.next_factory, std::forward<F3>(f3));
+    Construct(&prior.next_factory, std::forward<F4>(f4));
+  }
+  ~SeqState() {
+    switch (state) {
+      case State::kState0:
+        Destruct(&prior.prior.prior.prior.prior.current_promise);
+        goto tail0;
+      case State::kState1:
+        Destruct(&prior.prior.prior.prior.current_promise);
+        goto tail1;
+      case State::kState2:
+        Destruct(&prior.prior.prior.current_promise);
+        goto tail2;
+      case State::kState3:
+        Destruct(&prior.prior.current_promise);
+        goto tail3;
+      case State::kState4:
+        Destruct(&prior.current_promise);
+        goto tail4;
+      case State::kState5:
+        Destruct(&current_promise);
+        return;
+    }
+  tail0:
+    Destruct(&prior.prior.prior.prior.prior.next_factory);
+  tail1:
+    Destruct(&prior.prior.prior.prior.next_factory);
+  tail2:
+    Destruct(&prior.prior.prior.next_factory);
+  tail3:
+    Destruct(&prior.prior.next_factory);
+  tail4:
+    Destruct(&prior.next_factory);
+  }
+  SeqState(const SeqState& other) noexcept : state(other.state) {
+    GPR_ASSERT(state == State::kState0);
+    Construct(&prior.current_promise, other.prior.current_promise);
+    Construct(&prior.prior.prior.prior.prior.next_factory,
+              other.prior.prior.prior.prior.prior.next_factory);
+    Construct(&prior.prior.prior.prior.next_factory,
+              other.prior.prior.prior.prior.next_factory);
+    Construct(&prior.prior.prior.next_factory,
+              other.prior.prior.prior.next_factory);
+    Construct(&prior.prior.next_factory, other.prior.prior.next_factory);
+    Construct(&prior.next_factory, other.prior.next_factory);
+  }
+  SeqState& operator=(const SeqState& other) = delete;
+  SeqState(SeqState&& other) noexcept : state(other.state) {
+    switch (state) {
+      case State::kState0:
+        Construct(
+            &prior.prior.prior.prior.prior.current_promise,
+            std::move(other.prior.prior.prior.prior.prior.current_promise));
+        goto tail0;
+      case State::kState1:
+        Construct(&prior.prior.prior.prior.current_promise,
+                  std::move(other.prior.prior.prior.prior.current_promise));
+        goto tail1;
+      case State::kState2:
+        Construct(&prior.prior.prior.current_promise,
+                  std::move(other.prior.prior.prior.current_promise));
+        goto tail2;
+      case State::kState3:
+        Construct(&prior.prior.current_promise,
+                  std::move(other.prior.prior.current_promise));
+        goto tail3;
+      case State::kState4:
+        Construct(&prior.current_promise,
+                  std::move(other.prior.current_promise));
+        goto tail4;
+      case State::kState5:
+        Construct(&current_promise, std::move(other.current_promise));
+        return;
+    }
+  tail0:
+    Construct(&prior.prior.prior.prior.prior.next_factory,
+              std::move(other.prior.prior.prior.prior.prior.next_factory));
+  tail1:
+    Construct(&prior.prior.prior.prior.next_factory,
+              std::move(other.prior.prior.prior.prior.next_factory));
+  tail2:
+    Construct(&prior.prior.prior.next_factory,
+              std::move(other.prior.prior.prior.next_factory));
+  tail3:
+    Construct(&prior.prior.next_factory,
+              std::move(other.prior.prior.next_factory));
+  tail4:
+    Construct(&prior.next_factory, std::move(other.prior.next_factory));
+  }
+  SeqState& operator=(SeqState&& other) = delete;
+  Poll<Result> PollOnce() {
+    switch (state) {
+      case State::kState0: {
+        auto result = prior.prior.prior.prior.prior.current_promise();
+        PromiseResult0* p = result.value_if_ready();
+        if (p == nullptr) return Pending{};
+        if (!PromiseResultTraits0::IsOk(*p)) {
+          return PromiseResultTraits0::template ReturnValue<Result>(
+              std::move(*p));
+        }
+        Destruct(&prior.prior.prior.prior.prior.current_promise);
+        auto next_promise = PromiseResultTraits0::CallFactory(
+            &prior.prior.prior.prior.prior.next_factory, std::move(*p));
+        Destruct(&prior.prior.prior.prior.prior.next_factory);
+        Construct(&prior.prior.prior.prior.current_promise,
+                  std::move(next_promise));
+        state = State::kState1;
+      }
+        ABSL_FALLTHROUGH_INTENDED;
+      case State::kState1: {
+        auto result = prior.prior.prior.prior.current_promise();
+        PromiseResult1* p = result.value_if_ready();
+        if (p == nullptr) return Pending{};
+        if (!PromiseResultTraits1::IsOk(*p)) {
+          return PromiseResultTraits1::template ReturnValue<Result>(
+              std::move(*p));
+        }
+        Destruct(&prior.prior.prior.prior.current_promise);
+        auto next_promise = PromiseResultTraits1::CallFactory(
+            &prior.prior.prior.prior.next_factory, std::move(*p));
+        Destruct(&prior.prior.prior.prior.next_factory);
+        Construct(&prior.prior.prior.current_promise, std::move(next_promise));
+        state = State::kState2;
+      }
+        ABSL_FALLTHROUGH_INTENDED;
+      case State::kState2: {
+        auto result = prior.prior.prior.current_promise();
+        PromiseResult2* p = result.value_if_ready();
+        if (p == nullptr) return Pending{};
+        if (!PromiseResultTraits2::IsOk(*p)) {
+          return PromiseResultTraits2::template ReturnValue<Result>(
+              std::move(*p));
+        }
+        Destruct(&prior.prior.prior.current_promise);
+        auto next_promise = PromiseResultTraits2::CallFactory(
+            &prior.prior.prior.next_factory, std::move(*p));
+        Destruct(&prior.prior.prior.next_factory);
+        Construct(&prior.prior.current_promise, std::move(next_promise));
+        state = State::kState3;
+      }
+        ABSL_FALLTHROUGH_INTENDED;
+      case State::kState3: {
+        auto result = prior.prior.current_promise();
+        PromiseResult3* p = result.value_if_ready();
+        if (p == nullptr) return Pending{};
+        if (!PromiseResultTraits3::IsOk(*p)) {
+          return PromiseResultTraits3::template ReturnValue<Result>(
+              std::move(*p));
+        }
+        Destruct(&prior.prior.current_promise);
+        auto next_promise = PromiseResultTraits3::CallFactory(
+            &prior.prior.next_factory, std::move(*p));
+        Destruct(&prior.prior.next_factory);
+        Construct(&prior.current_promise, std::move(next_promise));
+        state = State::kState4;
+      }
+        ABSL_FALLTHROUGH_INTENDED;
+      case State::kState4: {
+        auto result = prior.current_promise();
+        PromiseResult4* p = result.value_if_ready();
+        if (p == nullptr) return Pending{};
+        if (!PromiseResultTraits4::IsOk(*p)) {
+          return PromiseResultTraits4::template ReturnValue<Result>(
+              std::move(*p));
+        }
+        Destruct(&prior.current_promise);
+        auto next_promise = PromiseResultTraits4::CallFactory(
+            &prior.next_factory, std::move(*p));
+        Destruct(&prior.next_factory);
+        Construct(&current_promise, std::move(next_promise));
+        state = State::kState5;
+      }
+        ABSL_FALLTHROUGH_INTENDED;
+      default:
+      case State::kState5: {
+        auto result = current_promise();
+        auto* p = result.value_if_ready();
+        if (p == nullptr) return Pending{};
+        return Result(std::move(*p));
+      }
+    }
+  }
+};
+
+template <template <typename> class Traits, typename P, typename F0,
+          typename F1, typename F2, typename F3, typename F4, typename F5>
+struct SeqState<Traits, P, F0, F1, F2, F3, F4, F5> {
+  using Promise0 = PromiseLike<P>;
+  using PromiseResult0 = typename Promise0::Result;
+  using PromiseResultTraits0 = Traits<PromiseResult0>;
+  using NextFactory0 =
+      OncePromiseFactory<typename PromiseResultTraits0::UnwrappedType, F0>;
+  using Promise1 = typename NextFactory0::Promise;
+  using PromiseResult1 = typename Promise1::Result;
+  using PromiseResultTraits1 = Traits<PromiseResult1>;
+  using NextFactory1 =
+      OncePromiseFactory<typename PromiseResultTraits1::UnwrappedType, F1>;
+  using Promise2 = typename NextFactory1::Promise;
+  using PromiseResult2 = typename Promise2::Result;
+  using PromiseResultTraits2 = Traits<PromiseResult2>;
+  using NextFactory2 =
+      OncePromiseFactory<typename PromiseResultTraits2::UnwrappedType, F2>;
+  using Promise3 = typename NextFactory2::Promise;
+  using PromiseResult3 = typename Promise3::Result;
+  using PromiseResultTraits3 = Traits<PromiseResult3>;
+  using NextFactory3 =
+      OncePromiseFactory<typename PromiseResultTraits3::UnwrappedType, F3>;
+  using Promise4 = typename NextFactory3::Promise;
+  using PromiseResult4 = typename Promise4::Result;
+  using PromiseResultTraits4 = Traits<PromiseResult4>;
+  using NextFactory4 =
+      OncePromiseFactory<typename PromiseResultTraits4::UnwrappedType, F4>;
+  using Promise5 = typename NextFactory4::Promise;
+  using PromiseResult5 = typename Promise5::Result;
+  using PromiseResultTraits5 = Traits<PromiseResult5>;
+  using NextFactory5 =
+      OncePromiseFactory<typename PromiseResultTraits5::UnwrappedType, F5>;
+  using Promise6 = typename NextFactory5::Promise;
+  using PromiseResult6 = typename Promise6::Result;
+  using PromiseResultTraits6 = Traits<PromiseResult6>;
+  using Result = typename PromiseResultTraits6::WrappedType;
+  struct Running0 {
+    GPR_NO_UNIQUE_ADDRESS Promise0 current_promise;
+    GPR_NO_UNIQUE_ADDRESS NextFactory0 next_factory;
+  };
+  struct Running1 {
+    union {
+      GPR_NO_UNIQUE_ADDRESS Running0 prior;
+      GPR_NO_UNIQUE_ADDRESS Promise1 current_promise;
+    };
+    GPR_NO_UNIQUE_ADDRESS NextFactory1 next_factory;
+  };
+  struct Running2 {
+    union {
+      GPR_NO_UNIQUE_ADDRESS Running1 prior;
+      GPR_NO_UNIQUE_ADDRESS Promise2 current_promise;
+    };
+    GPR_NO_UNIQUE_ADDRESS NextFactory2 next_factory;
+  };
+  struct Running3 {
+    union {
+      GPR_NO_UNIQUE_ADDRESS Running2 prior;
+      GPR_NO_UNIQUE_ADDRESS Promise3 current_promise;
+    };
+    GPR_NO_UNIQUE_ADDRESS NextFactory3 next_factory;
+  };
+  struct Running4 {
+    union {
+      GPR_NO_UNIQUE_ADDRESS Running3 prior;
+      GPR_NO_UNIQUE_ADDRESS Promise4 current_promise;
+    };
+    GPR_NO_UNIQUE_ADDRESS NextFactory4 next_factory;
+  };
+  struct Running5 {
+    union {
+      GPR_NO_UNIQUE_ADDRESS Running4 prior;
+      GPR_NO_UNIQUE_ADDRESS Promise5 current_promise;
+    };
+    GPR_NO_UNIQUE_ADDRESS NextFactory5 next_factory;
+  };
+  union {
+    GPR_NO_UNIQUE_ADDRESS Running5 prior;
+    GPR_NO_UNIQUE_ADDRESS Promise6 current_promise;
+  };
+  enum class State : uint8_t {
+    kState0,
+    kState1,
+    kState2,
+    kState3,
+    kState4,
+    kState5,
+    kState6
+  };
+  GPR_NO_UNIQUE_ADDRESS State state = State::kState0;
+
+  SeqState(P&& p, F0&& f0, F1&& f1, F2&& f2, F3&& f3, F4&& f4,
+           F5&& f5) noexcept {
+    Construct(&prior.prior.prior.prior.prior.prior.current_promise,
+              std::forward<P>(p));
+    Construct(&prior.prior.prior.prior.prior.prior.next_factory,
+              std::forward<F0>(f0));
+    Construct(&prior.prior.prior.prior.prior.next_factory,
+              std::forward<F1>(f1));
+    Construct(&prior.prior.prior.prior.next_factory, std::forward<F2>(f2));
+    Construct(&prior.prior.prior.next_factory, std::forward<F3>(f3));
+    Construct(&prior.prior.next_factory, std::forward<F4>(f4));
+    Construct(&prior.next_factory, std::forward<F5>(f5));
+  }
+  ~SeqState() {
+    switch (state) {
+      case State::kState0:
+        Destruct(&prior.prior.prior.prior.prior.prior.current_promise);
+        goto tail0;
+      case State::kState1:
+        Destruct(&prior.prior.prior.prior.prior.current_promise);
+        goto tail1;
+      case State::kState2:
+        Destruct(&prior.prior.prior.prior.current_promise);
+        goto tail2;
+      case State::kState3:
+        Destruct(&prior.prior.prior.current_promise);
+        goto tail3;
+      case State::kState4:
+        Destruct(&prior.prior.current_promise);
+        goto tail4;
+      case State::kState5:
+        Destruct(&prior.current_promise);
+        goto tail5;
+      case State::kState6:
+        Destruct(&current_promise);
+        return;
+    }
+  tail0:
+    Destruct(&prior.prior.prior.prior.prior.prior.next_factory);
+  tail1:
+    Destruct(&prior.prior.prior.prior.prior.next_factory);
+  tail2:
+    Destruct(&prior.prior.prior.prior.next_factory);
+  tail3:
+    Destruct(&prior.prior.prior.next_factory);
+  tail4:
+    Destruct(&prior.prior.next_factory);
+  tail5:
+    Destruct(&prior.next_factory);
+  }
+  SeqState(const SeqState& other) noexcept : state(other.state) {
+    GPR_ASSERT(state == State::kState0);
+    Construct(&prior.current_promise, other.prior.current_promise);
+    Construct(&prior.prior.prior.prior.prior.prior.next_factory,
+              other.prior.prior.prior.prior.prior.prior.next_factory);
+    Construct(&prior.prior.prior.prior.prior.next_factory,
+              other.prior.prior.prior.prior.prior.next_factory);
+    Construct(&prior.prior.prior.prior.next_factory,
+              other.prior.prior.prior.prior.next_factory);
+    Construct(&prior.prior.prior.next_factory,
+              other.prior.prior.prior.next_factory);
+    Construct(&prior.prior.next_factory, other.prior.prior.next_factory);
+    Construct(&prior.next_factory, other.prior.next_factory);
+  }
+  SeqState& operator=(const SeqState& other) = delete;
+  SeqState(SeqState&& other) noexcept : state(other.state) {
+    switch (state) {
+      case State::kState0:
+        Construct(
+            &prior.prior.prior.prior.prior.prior.current_promise,
+            std::move(
+                other.prior.prior.prior.prior.prior.prior.current_promise));
+        goto tail0;
+      case State::kState1:
+        Construct(
+            &prior.prior.prior.prior.prior.current_promise,
+            std::move(other.prior.prior.prior.prior.prior.current_promise));
+        goto tail1;
+      case State::kState2:
+        Construct(&prior.prior.prior.prior.current_promise,
+                  std::move(other.prior.prior.prior.prior.current_promise));
+        goto tail2;
+      case State::kState3:
+        Construct(&prior.prior.prior.current_promise,
+                  std::move(other.prior.prior.prior.current_promise));
+        goto tail3;
+      case State::kState4:
+        Construct(&prior.prior.current_promise,
+                  std::move(other.prior.prior.current_promise));
+        goto tail4;
+      case State::kState5:
+        Construct(&prior.current_promise,
+                  std::move(other.prior.current_promise));
+        goto tail5;
+      case State::kState6:
+        Construct(&current_promise, std::move(other.current_promise));
+        return;
+    }
+  tail0:
+    Construct(
+        &prior.prior.prior.prior.prior.prior.next_factory,
+        std::move(other.prior.prior.prior.prior.prior.prior.next_factory));
+  tail1:
+    Construct(&prior.prior.prior.prior.prior.next_factory,
+              std::move(other.prior.prior.prior.prior.prior.next_factory));
+  tail2:
+    Construct(&prior.prior.prior.prior.next_factory,
+              std::move(other.prior.prior.prior.prior.next_factory));
+  tail3:
+    Construct(&prior.prior.prior.next_factory,
+              std::move(other.prior.prior.prior.next_factory));
+  tail4:
+    Construct(&prior.prior.next_factory,
+              std::move(other.prior.prior.next_factory));
+  tail5:
+    Construct(&prior.next_factory, std::move(other.prior.next_factory));
+  }
+  SeqState& operator=(SeqState&& other) = delete;
+  Poll<Result> PollOnce() {
+    switch (state) {
+      case State::kState0: {
+        auto result = prior.prior.prior.prior.prior.prior.current_promise();
+        PromiseResult0* p = result.value_if_ready();
+        if (p == nullptr) return Pending{};
+        if (!PromiseResultTraits0::IsOk(*p)) {
+          return PromiseResultTraits0::template ReturnValue<Result>(
+              std::move(*p));
+        }
+        Destruct(&prior.prior.prior.prior.prior.prior.current_promise);
+        auto next_promise = PromiseResultTraits0::CallFactory(
+            &prior.prior.prior.prior.prior.prior.next_factory, std::move(*p));
+        Destruct(&prior.prior.prior.prior.prior.prior.next_factory);
+        Construct(&prior.prior.prior.prior.prior.current_promise,
+                  std::move(next_promise));
+        state = State::kState1;
+      }
+        ABSL_FALLTHROUGH_INTENDED;
+      case State::kState1: {
+        auto result = prior.prior.prior.prior.prior.current_promise();
+        PromiseResult1* p = result.value_if_ready();
+        if (p == nullptr) return Pending{};
+        if (!PromiseResultTraits1::IsOk(*p)) {
+          return PromiseResultTraits1::template ReturnValue<Result>(
+              std::move(*p));
+        }
+        Destruct(&prior.prior.prior.prior.prior.current_promise);
+        auto next_promise = PromiseResultTraits1::CallFactory(
+            &prior.prior.prior.prior.prior.next_factory, std::move(*p));
+        Destruct(&prior.prior.prior.prior.prior.next_factory);
+        Construct(&prior.prior.prior.prior.current_promise,
+                  std::move(next_promise));
+        state = State::kState2;
+      }
+        ABSL_FALLTHROUGH_INTENDED;
+      case State::kState2: {
+        auto result = prior.prior.prior.prior.current_promise();
+        PromiseResult2* p = result.value_if_ready();
+        if (p == nullptr) return Pending{};
+        if (!PromiseResultTraits2::IsOk(*p)) {
+          return PromiseResultTraits2::template ReturnValue<Result>(
+              std::move(*p));
+        }
+        Destruct(&prior.prior.prior.prior.current_promise);
+        auto next_promise = PromiseResultTraits2::CallFactory(
+            &prior.prior.prior.prior.next_factory, std::move(*p));
+        Destruct(&prior.prior.prior.prior.next_factory);
+        Construct(&prior.prior.prior.current_promise, std::move(next_promise));
+        state = State::kState3;
+      }
+        ABSL_FALLTHROUGH_INTENDED;
+      case State::kState3: {
+        auto result = prior.prior.prior.current_promise();
+        PromiseResult3* p = result.value_if_ready();
+        if (p == nullptr) return Pending{};
+        if (!PromiseResultTraits3::IsOk(*p)) {
+          return PromiseResultTraits3::template ReturnValue<Result>(
+              std::move(*p));
+        }
+        Destruct(&prior.prior.prior.current_promise);
+        auto next_promise = PromiseResultTraits3::CallFactory(
+            &prior.prior.prior.next_factory, std::move(*p));
+        Destruct(&prior.prior.prior.next_factory);
+        Construct(&prior.prior.current_promise, std::move(next_promise));
+        state = State::kState4;
+      }
+        ABSL_FALLTHROUGH_INTENDED;
+      case State::kState4: {
+        auto result = prior.prior.current_promise();
+        PromiseResult4* p = result.value_if_ready();
+        if (p == nullptr) return Pending{};
+        if (!PromiseResultTraits4::IsOk(*p)) {
+          return PromiseResultTraits4::template ReturnValue<Result>(
+              std::move(*p));
+        }
+        Destruct(&prior.prior.current_promise);
+        auto next_promise = PromiseResultTraits4::CallFactory(
+            &prior.prior.next_factory, std::move(*p));
+        Destruct(&prior.prior.next_factory);
+        Construct(&prior.current_promise, std::move(next_promise));
+        state = State::kState5;
+      }
+        ABSL_FALLTHROUGH_INTENDED;
+      case State::kState5: {
+        auto result = prior.current_promise();
+        PromiseResult5* p = result.value_if_ready();
+        if (p == nullptr) return Pending{};
+        if (!PromiseResultTraits5::IsOk(*p)) {
+          return PromiseResultTraits5::template ReturnValue<Result>(
+              std::move(*p));
+        }
+        Destruct(&prior.current_promise);
+        auto next_promise = PromiseResultTraits5::CallFactory(
+            &prior.next_factory, std::move(*p));
+        Destruct(&prior.next_factory);
+        Construct(&current_promise, std::move(next_promise));
+        state = State::kState6;
+      }
+        ABSL_FALLTHROUGH_INTENDED;
+      default:
+      case State::kState6: {
+        auto result = current_promise();
+        auto* p = result.value_if_ready();
+        if (p == nullptr) return Pending{};
+        return Result(std::move(*p));
+      }
+    }
+  }
+};
+
+template <template <typename> class Traits, typename P, typename F0,
+          typename F1, typename F2, typename F3, typename F4, typename F5,
+          typename F6>
+struct SeqState<Traits, P, F0, F1, F2, F3, F4, F5, F6> {
+  using Promise0 = PromiseLike<P>;
+  using PromiseResult0 = typename Promise0::Result;
+  using PromiseResultTraits0 = Traits<PromiseResult0>;
+  using NextFactory0 =
+      OncePromiseFactory<typename PromiseResultTraits0::UnwrappedType, F0>;
+  using Promise1 = typename NextFactory0::Promise;
+  using PromiseResult1 = typename Promise1::Result;
+  using PromiseResultTraits1 = Traits<PromiseResult1>;
+  using NextFactory1 =
+      OncePromiseFactory<typename PromiseResultTraits1::UnwrappedType, F1>;
+  using Promise2 = typename NextFactory1::Promise;
+  using PromiseResult2 = typename Promise2::Result;
+  using PromiseResultTraits2 = Traits<PromiseResult2>;
+  using NextFactory2 =
+      OncePromiseFactory<typename PromiseResultTraits2::UnwrappedType, F2>;
+  using Promise3 = typename NextFactory2::Promise;
+  using PromiseResult3 = typename Promise3::Result;
+  using PromiseResultTraits3 = Traits<PromiseResult3>;
+  using NextFactory3 =
+      OncePromiseFactory<typename PromiseResultTraits3::UnwrappedType, F3>;
+  using Promise4 = typename NextFactory3::Promise;
+  using PromiseResult4 = typename Promise4::Result;
+  using PromiseResultTraits4 = Traits<PromiseResult4>;
+  using NextFactory4 =
+      OncePromiseFactory<typename PromiseResultTraits4::UnwrappedType, F4>;
+  using Promise5 = typename NextFactory4::Promise;
+  using PromiseResult5 = typename Promise5::Result;
+  using PromiseResultTraits5 = Traits<PromiseResult5>;
+  using NextFactory5 =
+      OncePromiseFactory<typename PromiseResultTraits5::UnwrappedType, F5>;
+  using Promise6 = typename NextFactory5::Promise;
+  using PromiseResult6 = typename Promise6::Result;
+  using PromiseResultTraits6 = Traits<PromiseResult6>;
+  using NextFactory6 =
+      OncePromiseFactory<typename PromiseResultTraits6::UnwrappedType, F6>;
+  using Promise7 = typename NextFactory6::Promise;
+  using PromiseResult7 = typename Promise7::Result;
+  using PromiseResultTraits7 = Traits<PromiseResult7>;
+  using Result = typename PromiseResultTraits7::WrappedType;
+  struct Running0 {
+    GPR_NO_UNIQUE_ADDRESS Promise0 current_promise;
+    GPR_NO_UNIQUE_ADDRESS NextFactory0 next_factory;
+  };
+  struct Running1 {
+    union {
+      GPR_NO_UNIQUE_ADDRESS Running0 prior;
+      GPR_NO_UNIQUE_ADDRESS Promise1 current_promise;
+    };
+    GPR_NO_UNIQUE_ADDRESS NextFactory1 next_factory;
+  };
+  struct Running2 {
+    union {
+      GPR_NO_UNIQUE_ADDRESS Running1 prior;
+      GPR_NO_UNIQUE_ADDRESS Promise2 current_promise;
+    };
+    GPR_NO_UNIQUE_ADDRESS NextFactory2 next_factory;
+  };
+  struct Running3 {
+    union {
+      GPR_NO_UNIQUE_ADDRESS Running2 prior;
+      GPR_NO_UNIQUE_ADDRESS Promise3 current_promise;
+    };
+    GPR_NO_UNIQUE_ADDRESS NextFactory3 next_factory;
+  };
+  struct Running4 {
+    union {
+      GPR_NO_UNIQUE_ADDRESS Running3 prior;
+      GPR_NO_UNIQUE_ADDRESS Promise4 current_promise;
+    };
+    GPR_NO_UNIQUE_ADDRESS NextFactory4 next_factory;
+  };
+  struct Running5 {
+    union {
+      GPR_NO_UNIQUE_ADDRESS Running4 prior;
+      GPR_NO_UNIQUE_ADDRESS Promise5 current_promise;
+    };
+    GPR_NO_UNIQUE_ADDRESS NextFactory5 next_factory;
+  };
+  struct Running6 {
+    union {
+      GPR_NO_UNIQUE_ADDRESS Running5 prior;
+      GPR_NO_UNIQUE_ADDRESS Promise6 current_promise;
+    };
+    GPR_NO_UNIQUE_ADDRESS NextFactory6 next_factory;
+  };
+  union {
+    GPR_NO_UNIQUE_ADDRESS Running6 prior;
+    GPR_NO_UNIQUE_ADDRESS Promise7 current_promise;
+  };
+  enum class State : uint8_t {
+    kState0,
+    kState1,
+    kState2,
+    kState3,
+    kState4,
+    kState5,
+    kState6,
+    kState7
+  };
+  GPR_NO_UNIQUE_ADDRESS State state = State::kState0;
+
+  SeqState(P&& p, F0&& f0, F1&& f1, F2&& f2, F3&& f3, F4&& f4, F5&& f5,
+           F6&& f6) noexcept {
+    Construct(&prior.prior.prior.prior.prior.prior.prior.current_promise,
+              std::forward<P>(p));
+    Construct(&prior.prior.prior.prior.prior.prior.prior.next_factory,
+              std::forward<F0>(f0));
+    Construct(&prior.prior.prior.prior.prior.prior.next_factory,
+              std::forward<F1>(f1));
+    Construct(&prior.prior.prior.prior.prior.next_factory,
+              std::forward<F2>(f2));
+    Construct(&prior.prior.prior.prior.next_factory, std::forward<F3>(f3));
+    Construct(&prior.prior.prior.next_factory, std::forward<F4>(f4));
+    Construct(&prior.prior.next_factory, std::forward<F5>(f5));
+    Construct(&prior.next_factory, std::forward<F6>(f6));
+  }
+  ~SeqState() {
+    switch (state) {
+      case State::kState0:
+        Destruct(&prior.prior.prior.prior.prior.prior.prior.current_promise);
+        goto tail0;
+      case State::kState1:
+        Destruct(&prior.prior.prior.prior.prior.prior.current_promise);
+        goto tail1;
+      case State::kState2:
+        Destruct(&prior.prior.prior.prior.prior.current_promise);
+        goto tail2;
+      case State::kState3:
+        Destruct(&prior.prior.prior.prior.current_promise);
+        goto tail3;
+      case State::kState4:
+        Destruct(&prior.prior.prior.current_promise);
+        goto tail4;
+      case State::kState5:
+        Destruct(&prior.prior.current_promise);
+        goto tail5;
+      case State::kState6:
+        Destruct(&prior.current_promise);
+        goto tail6;
+      case State::kState7:
+        Destruct(&current_promise);
+        return;
+    }
+  tail0:
+    Destruct(&prior.prior.prior.prior.prior.prior.prior.next_factory);
+  tail1:
+    Destruct(&prior.prior.prior.prior.prior.prior.next_factory);
+  tail2:
+    Destruct(&prior.prior.prior.prior.prior.next_factory);
+  tail3:
+    Destruct(&prior.prior.prior.prior.next_factory);
+  tail4:
+    Destruct(&prior.prior.prior.next_factory);
+  tail5:
+    Destruct(&prior.prior.next_factory);
+  tail6:
+    Destruct(&prior.next_factory);
+  }
+  SeqState(const SeqState& other) noexcept : state(other.state) {
+    GPR_ASSERT(state == State::kState0);
+    Construct(&prior.current_promise, other.prior.current_promise);
+    Construct(&prior.prior.prior.prior.prior.prior.prior.next_factory,
+              other.prior.prior.prior.prior.prior.prior.prior.next_factory);
+    Construct(&prior.prior.prior.prior.prior.prior.next_factory,
+              other.prior.prior.prior.prior.prior.prior.next_factory);
+    Construct(&prior.prior.prior.prior.prior.next_factory,
+              other.prior.prior.prior.prior.prior.next_factory);
+    Construct(&prior.prior.prior.prior.next_factory,
+              other.prior.prior.prior.prior.next_factory);
+    Construct(&prior.prior.prior.next_factory,
+              other.prior.prior.prior.next_factory);
+    Construct(&prior.prior.next_factory, other.prior.prior.next_factory);
+    Construct(&prior.next_factory, other.prior.next_factory);
+  }
+  SeqState& operator=(const SeqState& other) = delete;
+  SeqState(SeqState&& other) noexcept : state(other.state) {
+    switch (state) {
+      case State::kState0:
+        Construct(&prior.prior.prior.prior.prior.prior.prior.current_promise,
+                  std::move(other.prior.prior.prior.prior.prior.prior.prior
+                                .current_promise));
+        goto tail0;
+      case State::kState1:
+        Construct(
+            &prior.prior.prior.prior.prior.prior.current_promise,
+            std::move(
+                other.prior.prior.prior.prior.prior.prior.current_promise));
+        goto tail1;
+      case State::kState2:
+        Construct(
+            &prior.prior.prior.prior.prior.current_promise,
+            std::move(other.prior.prior.prior.prior.prior.current_promise));
+        goto tail2;
+      case State::kState3:
+        Construct(&prior.prior.prior.prior.current_promise,
+                  std::move(other.prior.prior.prior.prior.current_promise));
+        goto tail3;
+      case State::kState4:
+        Construct(&prior.prior.prior.current_promise,
+                  std::move(other.prior.prior.prior.current_promise));
+        goto tail4;
+      case State::kState5:
+        Construct(&prior.prior.current_promise,
+                  std::move(other.prior.prior.current_promise));
+        goto tail5;
+      case State::kState6:
+        Construct(&prior.current_promise,
+                  std::move(other.prior.current_promise));
+        goto tail6;
+      case State::kState7:
+        Construct(&current_promise, std::move(other.current_promise));
+        return;
+    }
+  tail0:
+    Construct(
+        &prior.prior.prior.prior.prior.prior.prior.next_factory,
+        std::move(
+            other.prior.prior.prior.prior.prior.prior.prior.next_factory));
+  tail1:
+    Construct(
+        &prior.prior.prior.prior.prior.prior.next_factory,
+        std::move(other.prior.prior.prior.prior.prior.prior.next_factory));
+  tail2:
+    Construct(&prior.prior.prior.prior.prior.next_factory,
+              std::move(other.prior.prior.prior.prior.prior.next_factory));
+  tail3:
+    Construct(&prior.prior.prior.prior.next_factory,
+              std::move(other.prior.prior.prior.prior.next_factory));
+  tail4:
+    Construct(&prior.prior.prior.next_factory,
+              std::move(other.prior.prior.prior.next_factory));
+  tail5:
+    Construct(&prior.prior.next_factory,
+              std::move(other.prior.prior.next_factory));
+  tail6:
+    Construct(&prior.next_factory, std::move(other.prior.next_factory));
+  }
+  SeqState& operator=(SeqState&& other) = delete;
+  Poll<Result> PollOnce() {
+    switch (state) {
+      case State::kState0: {
+        auto result =
+            prior.prior.prior.prior.prior.prior.prior.current_promise();
+        PromiseResult0* p = result.value_if_ready();
+        if (p == nullptr) return Pending{};
+        if (!PromiseResultTraits0::IsOk(*p)) {
+          return PromiseResultTraits0::template ReturnValue<Result>(
+              std::move(*p));
+        }
+        Destruct(&prior.prior.prior.prior.prior.prior.prior.current_promise);
+        auto next_promise = PromiseResultTraits0::CallFactory(
+            &prior.prior.prior.prior.prior.prior.prior.next_factory,
+            std::move(*p));
+        Destruct(&prior.prior.prior.prior.prior.prior.prior.next_factory);
+        Construct(&prior.prior.prior.prior.prior.prior.current_promise,
+                  std::move(next_promise));
+        state = State::kState1;
+      }
+        ABSL_FALLTHROUGH_INTENDED;
+      case State::kState1: {
+        auto result = prior.prior.prior.prior.prior.prior.current_promise();
+        PromiseResult1* p = result.value_if_ready();
+        if (p == nullptr) return Pending{};
+        if (!PromiseResultTraits1::IsOk(*p)) {
+          return PromiseResultTraits1::template ReturnValue<Result>(
+              std::move(*p));
+        }
+        Destruct(&prior.prior.prior.prior.prior.prior.current_promise);
+        auto next_promise = PromiseResultTraits1::CallFactory(
+            &prior.prior.prior.prior.prior.prior.next_factory, std::move(*p));
+        Destruct(&prior.prior.prior.prior.prior.prior.next_factory);
+        Construct(&prior.prior.prior.prior.prior.current_promise,
+                  std::move(next_promise));
+        state = State::kState2;
+      }
+        ABSL_FALLTHROUGH_INTENDED;
+      case State::kState2: {
+        auto result = prior.prior.prior.prior.prior.current_promise();
+        PromiseResult2* p = result.value_if_ready();
+        if (p == nullptr) return Pending{};
+        if (!PromiseResultTraits2::IsOk(*p)) {
+          return PromiseResultTraits2::template ReturnValue<Result>(
+              std::move(*p));
+        }
+        Destruct(&prior.prior.prior.prior.prior.current_promise);
+        auto next_promise = PromiseResultTraits2::CallFactory(
+            &prior.prior.prior.prior.prior.next_factory, std::move(*p));
+        Destruct(&prior.prior.prior.prior.prior.next_factory);
+        Construct(&prior.prior.prior.prior.current_promise,
+                  std::move(next_promise));
+        state = State::kState3;
+      }
+        ABSL_FALLTHROUGH_INTENDED;
+      case State::kState3: {
+        auto result = prior.prior.prior.prior.current_promise();
+        PromiseResult3* p = result.value_if_ready();
+        if (p == nullptr) return Pending{};
+        if (!PromiseResultTraits3::IsOk(*p)) {
+          return PromiseResultTraits3::template ReturnValue<Result>(
+              std::move(*p));
+        }
+        Destruct(&prior.prior.prior.prior.current_promise);
+        auto next_promise = PromiseResultTraits3::CallFactory(
+            &prior.prior.prior.prior.next_factory, std::move(*p));
+        Destruct(&prior.prior.prior.prior.next_factory);
+        Construct(&prior.prior.prior.current_promise, std::move(next_promise));
+        state = State::kState4;
+      }
+        ABSL_FALLTHROUGH_INTENDED;
+      case State::kState4: {
+        auto result = prior.prior.prior.current_promise();
+        PromiseResult4* p = result.value_if_ready();
+        if (p == nullptr) return Pending{};
+        if (!PromiseResultTraits4::IsOk(*p)) {
+          return PromiseResultTraits4::template ReturnValue<Result>(
+              std::move(*p));
+        }
+        Destruct(&prior.prior.prior.current_promise);
+        auto next_promise = PromiseResultTraits4::CallFactory(
+            &prior.prior.prior.next_factory, std::move(*p));
+        Destruct(&prior.prior.prior.next_factory);
+        Construct(&prior.prior.current_promise, std::move(next_promise));
+        state = State::kState5;
+      }
+        ABSL_FALLTHROUGH_INTENDED;
+      case State::kState5: {
+        auto result = prior.prior.current_promise();
+        PromiseResult5* p = result.value_if_ready();
+        if (p == nullptr) return Pending{};
+        if (!PromiseResultTraits5::IsOk(*p)) {
+          return PromiseResultTraits5::template ReturnValue<Result>(
+              std::move(*p));
+        }
+        Destruct(&prior.prior.current_promise);
+        auto next_promise = PromiseResultTraits5::CallFactory(
+            &prior.prior.next_factory, std::move(*p));
+        Destruct(&prior.prior.next_factory);
+        Construct(&prior.current_promise, std::move(next_promise));
+        state = State::kState6;
+      }
+        ABSL_FALLTHROUGH_INTENDED;
+      case State::kState6: {
+        auto result = prior.current_promise();
+        PromiseResult6* p = result.value_if_ready();
+        if (p == nullptr) return Pending{};
+        if (!PromiseResultTraits6::IsOk(*p)) {
+          return PromiseResultTraits6::template ReturnValue<Result>(
+              std::move(*p));
+        }
+        Destruct(&prior.current_promise);
+        auto next_promise = PromiseResultTraits6::CallFactory(
+            &prior.next_factory, std::move(*p));
+        Destruct(&prior.next_factory);
+        Construct(&current_promise, std::move(next_promise));
+        state = State::kState7;
+      }
+        ABSL_FALLTHROUGH_INTENDED;
+      default:
+      case State::kState7: {
+        auto result = current_promise();
+        auto* p = result.value_if_ready();
+        if (p == nullptr) return Pending{};
+        return Result(std::move(*p));
+      }
+    }
+  }
+};
+
+template <template <typename> class Traits, typename P, typename F0,
+          typename F1, typename F2, typename F3, typename F4, typename F5,
+          typename F6, typename F7>
+struct SeqState<Traits, P, F0, F1, F2, F3, F4, F5, F6, F7> {
+  using Promise0 = PromiseLike<P>;
+  using PromiseResult0 = typename Promise0::Result;
+  using PromiseResultTraits0 = Traits<PromiseResult0>;
+  using NextFactory0 =
+      OncePromiseFactory<typename PromiseResultTraits0::UnwrappedType, F0>;
+  using Promise1 = typename NextFactory0::Promise;
+  using PromiseResult1 = typename Promise1::Result;
+  using PromiseResultTraits1 = Traits<PromiseResult1>;
+  using NextFactory1 =
+      OncePromiseFactory<typename PromiseResultTraits1::UnwrappedType, F1>;
+  using Promise2 = typename NextFactory1::Promise;
+  using PromiseResult2 = typename Promise2::Result;
+  using PromiseResultTraits2 = Traits<PromiseResult2>;
+  using NextFactory2 =
+      OncePromiseFactory<typename PromiseResultTraits2::UnwrappedType, F2>;
+  using Promise3 = typename NextFactory2::Promise;
+  using PromiseResult3 = typename Promise3::Result;
+  using PromiseResultTraits3 = Traits<PromiseResult3>;
+  using NextFactory3 =
+      OncePromiseFactory<typename PromiseResultTraits3::UnwrappedType, F3>;
+  using Promise4 = typename NextFactory3::Promise;
+  using PromiseResult4 = typename Promise4::Result;
+  using PromiseResultTraits4 = Traits<PromiseResult4>;
+  using NextFactory4 =
+      OncePromiseFactory<typename PromiseResultTraits4::UnwrappedType, F4>;
+  using Promise5 = typename NextFactory4::Promise;
+  using PromiseResult5 = typename Promise5::Result;
+  using PromiseResultTraits5 = Traits<PromiseResult5>;
+  using NextFactory5 =
+      OncePromiseFactory<typename PromiseResultTraits5::UnwrappedType, F5>;
+  using Promise6 = typename NextFactory5::Promise;
+  using PromiseResult6 = typename Promise6::Result;
+  using PromiseResultTraits6 = Traits<PromiseResult6>;
+  using NextFactory6 =
+      OncePromiseFactory<typename PromiseResultTraits6::UnwrappedType, F6>;
+  using Promise7 = typename NextFactory6::Promise;
+  using PromiseResult7 = typename Promise7::Result;
+  using PromiseResultTraits7 = Traits<PromiseResult7>;
+  using NextFactory7 =
+      OncePromiseFactory<typename PromiseResultTraits7::UnwrappedType, F7>;
+  using Promise8 = typename NextFactory7::Promise;
+  using PromiseResult8 = typename Promise8::Result;
+  using PromiseResultTraits8 = Traits<PromiseResult8>;
+  using Result = typename PromiseResultTraits8::WrappedType;
+  struct Running0 {
+    GPR_NO_UNIQUE_ADDRESS Promise0 current_promise;
+    GPR_NO_UNIQUE_ADDRESS NextFactory0 next_factory;
+  };
+  struct Running1 {
+    union {
+      GPR_NO_UNIQUE_ADDRESS Running0 prior;
+      GPR_NO_UNIQUE_ADDRESS Promise1 current_promise;
+    };
+    GPR_NO_UNIQUE_ADDRESS NextFactory1 next_factory;
+  };
+  struct Running2 {
+    union {
+      GPR_NO_UNIQUE_ADDRESS Running1 prior;
+      GPR_NO_UNIQUE_ADDRESS Promise2 current_promise;
+    };
+    GPR_NO_UNIQUE_ADDRESS NextFactory2 next_factory;
+  };
+  struct Running3 {
+    union {
+      GPR_NO_UNIQUE_ADDRESS Running2 prior;
+      GPR_NO_UNIQUE_ADDRESS Promise3 current_promise;
+    };
+    GPR_NO_UNIQUE_ADDRESS NextFactory3 next_factory;
+  };
+  struct Running4 {
+    union {
+      GPR_NO_UNIQUE_ADDRESS Running3 prior;
+      GPR_NO_UNIQUE_ADDRESS Promise4 current_promise;
+    };
+    GPR_NO_UNIQUE_ADDRESS NextFactory4 next_factory;
+  };
+  struct Running5 {
+    union {
+      GPR_NO_UNIQUE_ADDRESS Running4 prior;
+      GPR_NO_UNIQUE_ADDRESS Promise5 current_promise;
+    };
+    GPR_NO_UNIQUE_ADDRESS NextFactory5 next_factory;
+  };
+  struct Running6 {
+    union {
+      GPR_NO_UNIQUE_ADDRESS Running5 prior;
+      GPR_NO_UNIQUE_ADDRESS Promise6 current_promise;
+    };
+    GPR_NO_UNIQUE_ADDRESS NextFactory6 next_factory;
+  };
+  struct Running7 {
+    union {
+      GPR_NO_UNIQUE_ADDRESS Running6 prior;
+      GPR_NO_UNIQUE_ADDRESS Promise7 current_promise;
+    };
+    GPR_NO_UNIQUE_ADDRESS NextFactory7 next_factory;
+  };
+  union {
+    GPR_NO_UNIQUE_ADDRESS Running7 prior;
+    GPR_NO_UNIQUE_ADDRESS Promise8 current_promise;
+  };
+  enum class State : uint8_t {
+    kState0,
+    kState1,
+    kState2,
+    kState3,
+    kState4,
+    kState5,
+    kState6,
+    kState7,
+    kState8
+  };
+  GPR_NO_UNIQUE_ADDRESS State state = State::kState0;
+
+  SeqState(P&& p, F0&& f0, F1&& f1, F2&& f2, F3&& f3, F4&& f4, F5&& f5, F6&& f6,
+           F7&& f7) noexcept {
+    Construct(&prior.prior.prior.prior.prior.prior.prior.prior.current_promise,
+              std::forward<P>(p));
+    Construct(&prior.prior.prior.prior.prior.prior.prior.prior.next_factory,
+              std::forward<F0>(f0));
+    Construct(&prior.prior.prior.prior.prior.prior.prior.next_factory,
+              std::forward<F1>(f1));
+    Construct(&prior.prior.prior.prior.prior.prior.next_factory,
+              std::forward<F2>(f2));
+    Construct(&prior.prior.prior.prior.prior.next_factory,
+              std::forward<F3>(f3));
+    Construct(&prior.prior.prior.prior.next_factory, std::forward<F4>(f4));
+    Construct(&prior.prior.prior.next_factory, std::forward<F5>(f5));
+    Construct(&prior.prior.next_factory, std::forward<F6>(f6));
+    Construct(&prior.next_factory, std::forward<F7>(f7));
+  }
+  ~SeqState() {
+    switch (state) {
+      case State::kState0:
+        Destruct(
+            &prior.prior.prior.prior.prior.prior.prior.prior.current_promise);
+        goto tail0;
+      case State::kState1:
+        Destruct(&prior.prior.prior.prior.prior.prior.prior.current_promise);
+        goto tail1;
+      case State::kState2:
+        Destruct(&prior.prior.prior.prior.prior.prior.current_promise);
+        goto tail2;
+      case State::kState3:
+        Destruct(&prior.prior.prior.prior.prior.current_promise);
+        goto tail3;
+      case State::kState4:
+        Destruct(&prior.prior.prior.prior.current_promise);
+        goto tail4;
+      case State::kState5:
+        Destruct(&prior.prior.prior.current_promise);
+        goto tail5;
+      case State::kState6:
+        Destruct(&prior.prior.current_promise);
+        goto tail6;
+      case State::kState7:
+        Destruct(&prior.current_promise);
+        goto tail7;
+      case State::kState8:
+        Destruct(&current_promise);
+        return;
+    }
+  tail0:
+    Destruct(&prior.prior.prior.prior.prior.prior.prior.prior.next_factory);
+  tail1:
+    Destruct(&prior.prior.prior.prior.prior.prior.prior.next_factory);
+  tail2:
+    Destruct(&prior.prior.prior.prior.prior.prior.next_factory);
+  tail3:
+    Destruct(&prior.prior.prior.prior.prior.next_factory);
+  tail4:
+    Destruct(&prior.prior.prior.prior.next_factory);
+  tail5:
+    Destruct(&prior.prior.prior.next_factory);
+  tail6:
+    Destruct(&prior.prior.next_factory);
+  tail7:
+    Destruct(&prior.next_factory);
+  }
+  SeqState(const SeqState& other) noexcept : state(other.state) {
+    GPR_ASSERT(state == State::kState0);
+    Construct(&prior.current_promise, other.prior.current_promise);
+    Construct(
+        &prior.prior.prior.prior.prior.prior.prior.prior.next_factory,
+        other.prior.prior.prior.prior.prior.prior.prior.prior.next_factory);
+    Construct(&prior.prior.prior.prior.prior.prior.prior.next_factory,
+              other.prior.prior.prior.prior.prior.prior.prior.next_factory);
+    Construct(&prior.prior.prior.prior.prior.prior.next_factory,
+              other.prior.prior.prior.prior.prior.prior.next_factory);
+    Construct(&prior.prior.prior.prior.prior.next_factory,
+              other.prior.prior.prior.prior.prior.next_factory);
+    Construct(&prior.prior.prior.prior.next_factory,
+              other.prior.prior.prior.prior.next_factory);
+    Construct(&prior.prior.prior.next_factory,
+              other.prior.prior.prior.next_factory);
+    Construct(&prior.prior.next_factory, other.prior.prior.next_factory);
+    Construct(&prior.next_factory, other.prior.next_factory);
+  }
+  SeqState& operator=(const SeqState& other) = delete;
+  SeqState(SeqState&& other) noexcept : state(other.state) {
+    switch (state) {
+      case State::kState0:
+        Construct(
+            &prior.prior.prior.prior.prior.prior.prior.prior.current_promise,
+            std::move(other.prior.prior.prior.prior.prior.prior.prior.prior
+                          .current_promise));
+        goto tail0;
+      case State::kState1:
+        Construct(&prior.prior.prior.prior.prior.prior.prior.current_promise,
+                  std::move(other.prior.prior.prior.prior.prior.prior.prior
+                                .current_promise));
+        goto tail1;
+      case State::kState2:
+        Construct(
+            &prior.prior.prior.prior.prior.prior.current_promise,
+            std::move(
+                other.prior.prior.prior.prior.prior.prior.current_promise));
+        goto tail2;
+      case State::kState3:
+        Construct(
+            &prior.prior.prior.prior.prior.current_promise,
+            std::move(other.prior.prior.prior.prior.prior.current_promise));
+        goto tail3;
+      case State::kState4:
+        Construct(&prior.prior.prior.prior.current_promise,
+                  std::move(other.prior.prior.prior.prior.current_promise));
+        goto tail4;
+      case State::kState5:
+        Construct(&prior.prior.prior.current_promise,
+                  std::move(other.prior.prior.prior.current_promise));
+        goto tail5;
+      case State::kState6:
+        Construct(&prior.prior.current_promise,
+                  std::move(other.prior.prior.current_promise));
+        goto tail6;
+      case State::kState7:
+        Construct(&prior.current_promise,
+                  std::move(other.prior.current_promise));
+        goto tail7;
+      case State::kState8:
+        Construct(&current_promise, std::move(other.current_promise));
+        return;
+    }
+  tail0:
+    Construct(&prior.prior.prior.prior.prior.prior.prior.prior.next_factory,
+              std::move(other.prior.prior.prior.prior.prior.prior.prior.prior
+                            .next_factory));
+  tail1:
+    Construct(
+        &prior.prior.prior.prior.prior.prior.prior.next_factory,
+        std::move(
+            other.prior.prior.prior.prior.prior.prior.prior.next_factory));
+  tail2:
+    Construct(
+        &prior.prior.prior.prior.prior.prior.next_factory,
+        std::move(other.prior.prior.prior.prior.prior.prior.next_factory));
+  tail3:
+    Construct(&prior.prior.prior.prior.prior.next_factory,
+              std::move(other.prior.prior.prior.prior.prior.next_factory));
+  tail4:
+    Construct(&prior.prior.prior.prior.next_factory,
+              std::move(other.prior.prior.prior.prior.next_factory));
+  tail5:
+    Construct(&prior.prior.prior.next_factory,
+              std::move(other.prior.prior.prior.next_factory));
+  tail6:
+    Construct(&prior.prior.next_factory,
+              std::move(other.prior.prior.next_factory));
+  tail7:
+    Construct(&prior.next_factory, std::move(other.prior.next_factory));
+  }
+  SeqState& operator=(SeqState&& other) = delete;
+  Poll<Result> PollOnce() {
+    switch (state) {
+      case State::kState0: {
+        auto result =
+            prior.prior.prior.prior.prior.prior.prior.prior.current_promise();
+        PromiseResult0* p = result.value_if_ready();
+        if (p == nullptr) return Pending{};
+        if (!PromiseResultTraits0::IsOk(*p)) {
+          return PromiseResultTraits0::template ReturnValue<Result>(
+              std::move(*p));
+        }
+        Destruct(
+            &prior.prior.prior.prior.prior.prior.prior.prior.current_promise);
+        auto next_promise = PromiseResultTraits0::CallFactory(
+            &prior.prior.prior.prior.prior.prior.prior.prior.next_factory,
+            std::move(*p));
+        Destruct(&prior.prior.prior.prior.prior.prior.prior.prior.next_factory);
+        Construct(&prior.prior.prior.prior.prior.prior.prior.current_promise,
+                  std::move(next_promise));
+        state = State::kState1;
+      }
+        ABSL_FALLTHROUGH_INTENDED;
+      case State::kState1: {
+        auto result =
+            prior.prior.prior.prior.prior.prior.prior.current_promise();
+        PromiseResult1* p = result.value_if_ready();
+        if (p == nullptr) return Pending{};
+        if (!PromiseResultTraits1::IsOk(*p)) {
+          return PromiseResultTraits1::template ReturnValue<Result>(
+              std::move(*p));
+        }
+        Destruct(&prior.prior.prior.prior.prior.prior.prior.current_promise);
+        auto next_promise = PromiseResultTraits1::CallFactory(
+            &prior.prior.prior.prior.prior.prior.prior.next_factory,
+            std::move(*p));
+        Destruct(&prior.prior.prior.prior.prior.prior.prior.next_factory);
+        Construct(&prior.prior.prior.prior.prior.prior.current_promise,
+                  std::move(next_promise));
+        state = State::kState2;
+      }
+        ABSL_FALLTHROUGH_INTENDED;
+      case State::kState2: {
+        auto result = prior.prior.prior.prior.prior.prior.current_promise();
+        PromiseResult2* p = result.value_if_ready();
+        if (p == nullptr) return Pending{};
+        if (!PromiseResultTraits2::IsOk(*p)) {
+          return PromiseResultTraits2::template ReturnValue<Result>(
+              std::move(*p));
+        }
+        Destruct(&prior.prior.prior.prior.prior.prior.current_promise);
+        auto next_promise = PromiseResultTraits2::CallFactory(
+            &prior.prior.prior.prior.prior.prior.next_factory, std::move(*p));
+        Destruct(&prior.prior.prior.prior.prior.prior.next_factory);
+        Construct(&prior.prior.prior.prior.prior.current_promise,
+                  std::move(next_promise));
+        state = State::kState3;
+      }
+        ABSL_FALLTHROUGH_INTENDED;
+      case State::kState3: {
+        auto result = prior.prior.prior.prior.prior.current_promise();
+        PromiseResult3* p = result.value_if_ready();
+        if (p == nullptr) return Pending{};
+        if (!PromiseResultTraits3::IsOk(*p)) {
+          return PromiseResultTraits3::template ReturnValue<Result>(
+              std::move(*p));
+        }
+        Destruct(&prior.prior.prior.prior.prior.current_promise);
+        auto next_promise = PromiseResultTraits3::CallFactory(
+            &prior.prior.prior.prior.prior.next_factory, std::move(*p));
+        Destruct(&prior.prior.prior.prior.prior.next_factory);
+        Construct(&prior.prior.prior.prior.current_promise,
+                  std::move(next_promise));
+        state = State::kState4;
+      }
+        ABSL_FALLTHROUGH_INTENDED;
+      case State::kState4: {
+        auto result = prior.prior.prior.prior.current_promise();
+        PromiseResult4* p = result.value_if_ready();
+        if (p == nullptr) return Pending{};
+        if (!PromiseResultTraits4::IsOk(*p)) {
+          return PromiseResultTraits4::template ReturnValue<Result>(
+              std::move(*p));
+        }
+        Destruct(&prior.prior.prior.prior.current_promise);
+        auto next_promise = PromiseResultTraits4::CallFactory(
+            &prior.prior.prior.prior.next_factory, std::move(*p));
+        Destruct(&prior.prior.prior.prior.next_factory);
+        Construct(&prior.prior.prior.current_promise, std::move(next_promise));
+        state = State::kState5;
+      }
+        ABSL_FALLTHROUGH_INTENDED;
+      case State::kState5: {
+        auto result = prior.prior.prior.current_promise();
+        PromiseResult5* p = result.value_if_ready();
+        if (p == nullptr) return Pending{};
+        if (!PromiseResultTraits5::IsOk(*p)) {
+          return PromiseResultTraits5::template ReturnValue<Result>(
+              std::move(*p));
+        }
+        Destruct(&prior.prior.prior.current_promise);
+        auto next_promise = PromiseResultTraits5::CallFactory(
+            &prior.prior.prior.next_factory, std::move(*p));
+        Destruct(&prior.prior.prior.next_factory);
+        Construct(&prior.prior.current_promise, std::move(next_promise));
+        state = State::kState6;
+      }
+        ABSL_FALLTHROUGH_INTENDED;
+      case State::kState6: {
+        auto result = prior.prior.current_promise();
+        PromiseResult6* p = result.value_if_ready();
+        if (p == nullptr) return Pending{};
+        if (!PromiseResultTraits6::IsOk(*p)) {
+          return PromiseResultTraits6::template ReturnValue<Result>(
+              std::move(*p));
+        }
+        Destruct(&prior.prior.current_promise);
+        auto next_promise = PromiseResultTraits6::CallFactory(
+            &prior.prior.next_factory, std::move(*p));
+        Destruct(&prior.prior.next_factory);
+        Construct(&prior.current_promise, std::move(next_promise));
+        state = State::kState7;
+      }
+        ABSL_FALLTHROUGH_INTENDED;
+      case State::kState7: {
+        auto result = prior.current_promise();
+        PromiseResult7* p = result.value_if_ready();
+        if (p == nullptr) return Pending{};
+        if (!PromiseResultTraits7::IsOk(*p)) {
+          return PromiseResultTraits7::template ReturnValue<Result>(
+              std::move(*p));
+        }
+        Destruct(&prior.current_promise);
+        auto next_promise = PromiseResultTraits7::CallFactory(
+            &prior.next_factory, std::move(*p));
+        Destruct(&prior.next_factory);
+        Construct(&current_promise, std::move(next_promise));
+        state = State::kState8;
+      }
+        ABSL_FALLTHROUGH_INTENDED;
+      default:
+      case State::kState8: {
+        auto result = current_promise();
+        auto* p = result.value_if_ready();
+        if (p == nullptr) return Pending{};
+        return Result(std::move(*p));
+      }
+    }
+  }
+};
+
+}  // namespace promise_detail
+}  // namespace grpc_core
+
+#endif  // GRPC_SRC_CORE_LIB_PROMISE_DETAIL_SEQ_STATE_H
diff --git a/src/core/lib/promise/seq.h b/src/core/lib/promise/seq.h
index 9a7545418a9..70b8590def8 100644
--- a/src/core/lib/promise/seq.h
+++ b/src/core/lib/promise/seq.h
@@ -17,11 +17,14 @@
 
 #include <grpc/support/port_platform.h>
 
+#include <stdlib.h>
+
 #include <type_traits>
 #include <utility>
 
 #include "src/core/lib/promise/detail/basic_seq.h"
 #include "src/core/lib/promise/detail/promise_like.h"
+#include "src/core/lib/promise/detail/seq_state.h"
 #include "src/core/lib/promise/poll.h"
 
 namespace grpc_core {
@@ -36,6 +39,11 @@ struct SeqTraits {
   static auto CallFactory(Next* next, T&& value) {
     return next->Make(std::forward<T>(value));
   }
+  static bool IsOk(const T&) { return true; }
+  template <typename R>
+  static R ReturnValue(T&&) {
+    abort();
+  }
   template <typename F, typename Elem>
   static auto CallSeqFactory(F& f, Elem&& elem, T&& value) {
     return f(std::forward<Elem>(elem), std::forward<T>(value));
@@ -47,8 +55,17 @@ struct SeqTraits {
   }
 };
 
-template <typename... Fs>
-using Seq = BasicSeq<SeqTraits, Fs...>;
+template <typename P, typename... Fs>
+class Seq {
+ public:
+  explicit Seq(P&& promise, Fs&&... factories)
+      : state_(std::forward<P>(promise), std::forward<Fs>(factories)...) {}
+
+  auto operator()() { return state_.PollOnce(); }
+
+ private:
+  SeqState<SeqTraits, P, Fs...> state_;
+};
 
 template <typename I, typename F, typename Arg>
 struct SeqIterTraits {
diff --git a/src/core/lib/promise/try_seq.h b/src/core/lib/promise/try_seq.h
index be24335694f..71946e0e290 100644
--- a/src/core/lib/promise/try_seq.h
+++ b/src/core/lib/promise/try_seq.h
@@ -17,6 +17,8 @@
 
 #include <grpc/support/port_platform.h>
 
+#include <stdlib.h>
+
 #include <type_traits>
 #include <utility>
 
@@ -26,6 +28,7 @@
 
 #include "src/core/lib/promise/detail/basic_seq.h"
 #include "src/core/lib/promise/detail/promise_like.h"
+#include "src/core/lib/promise/detail/seq_state.h"
 #include "src/core/lib/promise/detail/status.h"
 #include "src/core/lib/promise/poll.h"
 
@@ -41,6 +44,11 @@ struct TrySeqTraitsWithSfinae {
   static auto CallFactory(Next* next, T&& value) {
     return next->Make(std::forward<T>(value));
   }
+  static bool IsOk(const T&) { return true; }
+  template <typename R>
+  static R ReturnValue(T&&) {
+    abort();
+  }
   template <typename F, typename Elem>
   static auto CallSeqFactory(F& f, Elem&& elem, T&& value)
       -> decltype(f(std::forward<Elem>(elem), std::forward<T>(value))) {
@@ -60,6 +68,11 @@ struct TrySeqTraitsWithSfinae<absl::StatusOr<T>> {
   static auto CallFactory(Next* next, absl::StatusOr<T>&& status) {
     return next->Make(std::move(*status));
   }
+  static bool IsOk(const absl::StatusOr<T>& status) { return status.ok(); }
+  template <typename R>
+  static R ReturnValue(absl::StatusOr<T>&& status) {
+    return StatusCast<R>(status.status());
+  }
   template <typename F, typename Elem>
   static auto CallSeqFactory(F& f, Elem&& elem, absl::StatusOr<T> value)
       -> decltype(f(std::forward<Elem>(elem), std::move(*value))) {
@@ -86,6 +99,11 @@ struct TrySeqTraitsWithSfinae<
   static auto CallFactory(Next* next, T&&) {
     return next->Make();
   }
+  static bool IsOk(const T& status) { return IsStatusOk(status); }
+  template <typename R>
+  static R ReturnValue(T&& status) {
+    return R(std::move(status));
+  }
   template <typename Result, typename RunNext>
   static Poll<Result> CheckResultAndRunNext(T prior, RunNext run_next) {
     if (!IsStatusOk(prior)) return Result(std::move(prior));
@@ -100,6 +118,11 @@ struct TrySeqTraitsWithSfinae<absl::Status> {
   static auto CallFactory(Next* next, absl::Status&&) {
     return next->Make();
   }
+  static bool IsOk(const absl::Status& status) { return status.ok(); }
+  template <typename R>
+  static R ReturnValue(absl::Status&& status) {
+    return StatusCast<R>(std::move(status));
+  }
   template <typename Result, typename RunNext>
   static Poll<Result> CheckResultAndRunNext(absl::Status prior,
                                             RunNext run_next) {
@@ -111,8 +134,17 @@ struct TrySeqTraitsWithSfinae<absl::Status> {
 template <typename T>
 using TrySeqTraits = TrySeqTraitsWithSfinae<T>;
 
-template <typename... Fs>
-using TrySeq = BasicSeq<TrySeqTraits, Fs...>;
+template <typename P, typename... Fs>
+class TrySeq {
+ public:
+  explicit TrySeq(P&& promise, Fs&&... factories)
+      : state_(std::forward<P>(promise), std::forward<Fs>(factories)...) {}
+
+  auto operator()() { return state_.PollOnce(); }
+
+ private:
+  SeqState<TrySeqTraits, P, Fs...> state_;
+};
 
 template <typename I, typename F, typename Arg>
 struct TrySeqIterTraits {
diff --git a/src/core/lib/resource_quota/memory_quota.cc b/src/core/lib/resource_quota/memory_quota.cc
index 2b4241ceccc..32b7ceda60f 100644
--- a/src/core/lib/resource_quota/memory_quota.cc
+++ b/src/core/lib/resource_quota/memory_quota.cc
@@ -28,7 +28,6 @@
 #include "src/core/lib/debug/trace.h"
 #include "src/core/lib/gpr/useful.h"
 #include "src/core/lib/gprpp/mpscq.h"
-#include "src/core/lib/promise/detail/basic_seq.h"
 #include "src/core/lib/promise/exec_ctx_wakeup_scheduler.h"
 #include "src/core/lib/promise/loop.h"
 #include "src/core/lib/promise/map.h"
diff --git a/src/core/lib/security/transport/client_auth_filter.cc b/src/core/lib/security/transport/client_auth_filter.cc
index 57d10f9a0b7..fa1e9f3bd92 100644
--- a/src/core/lib/security/transport/client_auth_filter.cc
+++ b/src/core/lib/security/transport/client_auth_filter.cc
@@ -18,6 +18,7 @@
 
 #include <grpc/support/port_platform.h>
 
+#include <stdint.h>
 #include <string.h>
 
 #include <functional>
@@ -43,7 +44,6 @@
 #include "src/core/lib/gprpp/ref_counted_ptr.h"
 #include "src/core/lib/promise/arena_promise.h"
 #include "src/core/lib/promise/context.h"
-#include "src/core/lib/promise/detail/basic_seq.h"
 #include "src/core/lib/promise/promise.h"
 #include "src/core/lib/promise/seq.h"
 #include "src/core/lib/promise/try_seq.h"
diff --git a/src/core/lib/security/transport/server_auth_filter.cc b/src/core/lib/security/transport/server_auth_filter.cc
index c67b97388ba..548eb6c271e 100644
--- a/src/core/lib/security/transport/server_auth_filter.cc
+++ b/src/core/lib/security/transport/server_auth_filter.cc
@@ -18,6 +18,8 @@
 
 #include <grpc/support/port_platform.h>
 
+#include <stdint.h>
+
 #include <algorithm>
 #include <atomic>
 #include <cstddef>
diff --git a/src/core/lib/surface/call.cc b/src/core/lib/surface/call.cc
index ee81abf5231..bb740d46a8e 100644
--- a/src/core/lib/surface/call.cc
+++ b/src/core/lib/surface/call.cc
@@ -85,7 +85,6 @@
 #include "src/core/lib/promise/activity.h"
 #include "src/core/lib/promise/arena_promise.h"
 #include "src/core/lib/promise/context.h"
-#include "src/core/lib/promise/detail/basic_seq.h"
 #include "src/core/lib/promise/latch.h"
 #include "src/core/lib/promise/map.h"
 #include "src/core/lib/promise/party.h"
diff --git a/src/core/lib/surface/server.cc b/src/core/lib/surface/server.cc
index a1d87eded0c..be829a0d57a 100644
--- a/src/core/lib/surface/server.cc
+++ b/src/core/lib/surface/server.cc
@@ -61,11 +61,11 @@
 #include "src/core/lib/iomgr/pollset_set.h"
 #include "src/core/lib/promise/activity.h"
 #include "src/core/lib/promise/context.h"
-#include "src/core/lib/promise/detail/basic_seq.h"
 #include "src/core/lib/promise/map.h"
 #include "src/core/lib/promise/pipe.h"
 #include "src/core/lib/promise/poll.h"
 #include "src/core/lib/promise/promise.h"
+#include "src/core/lib/promise/seq.h"
 #include "src/core/lib/promise/try_seq.h"
 #include "src/core/lib/slice/slice_buffer.h"
 #include "src/core/lib/slice/slice_internal.h"
diff --git a/test/core/filters/filter_test.cc b/test/core/filters/filter_test.cc
index 8b18254961d..738545212aa 100644
--- a/test/core/filters/filter_test.cc
+++ b/test/core/filters/filter_test.cc
@@ -31,9 +31,9 @@
 #include "src/core/lib/promise/activity.h"
 #include "src/core/lib/promise/arena_promise.h"
 #include "src/core/lib/promise/context.h"
-#include "src/core/lib/promise/detail/basic_seq.h"
 #include "src/core/lib/promise/pipe.h"
 #include "src/core/lib/promise/poll.h"
+#include "src/core/lib/promise/seq.h"
 #include "src/core/lib/resource_quota/arena.h"
 #include "src/core/lib/slice/slice.h"
 #include "test/core/event_engine/fuzzing_event_engine/fuzzing_event_engine.pb.h"
diff --git a/test/core/promise/BUILD b/test/core/promise/BUILD
index 620c40a2c65..1d6abd20238 100644
--- a/test/core/promise/BUILD
+++ b/test/core/promise/BUILD
@@ -206,7 +206,6 @@ grpc_cc_test(
     uses_event_engine = False,
     uses_polling = False,
     deps = [
-        "//src/core:basic_seq",
         "//src/core:loop",
         "//src/core:seq",
     ],
@@ -337,7 +336,6 @@ grpc_cc_test(
         "//src/core:activity",
         "//src/core:arena",
         "//src/core:basic_join",
-        "//src/core:basic_seq",
         "//src/core:event_engine_memory_allocator",
         "//src/core:for_each",
         "//src/core:join",
@@ -364,7 +362,6 @@ grpc_cc_test(
         "//src/core:activity",
         "//src/core:arena",
         "//src/core:basic_join",
-        "//src/core:basic_seq",
         "//src/core:event_engine_memory_allocator",
         "//src/core:for_each",
         "//src/core:join",
diff --git a/test/core/promise/for_each_test.cc b/test/core/promise/for_each_test.cc
index 16e677dfb77..017b0e211d2 100644
--- a/test/core/promise/for_each_test.cc
+++ b/test/core/promise/for_each_test.cc
@@ -27,7 +27,6 @@
 #include "src/core/lib/gprpp/ref_counted_ptr.h"
 #include "src/core/lib/promise/activity.h"
 #include "src/core/lib/promise/detail/basic_join.h"
-#include "src/core/lib/promise/detail/basic_seq.h"
 #include "src/core/lib/promise/join.h"
 #include "src/core/lib/promise/map.h"
 #include "src/core/lib/promise/pipe.h"
diff --git a/test/core/promise/loop_test.cc b/test/core/promise/loop_test.cc
index 30fa4d1dd95..9a8e619249b 100644
--- a/test/core/promise/loop_test.cc
+++ b/test/core/promise/loop_test.cc
@@ -14,11 +14,12 @@
 
 #include "src/core/lib/promise/loop.h"
 
+#include <stdint.h>
+
 #include <utility>
 
 #include "gtest/gtest.h"
 
-#include "src/core/lib/promise/detail/basic_seq.h"
 #include "src/core/lib/promise/seq.h"
 
 namespace grpc_core {
diff --git a/test/core/promise/map_pipe_test.cc b/test/core/promise/map_pipe_test.cc
index 4fe4654b456..5e30627938b 100644
--- a/test/core/promise/map_pipe_test.cc
+++ b/test/core/promise/map_pipe_test.cc
@@ -28,7 +28,6 @@
 #include "src/core/lib/gprpp/ref_counted_ptr.h"
 #include "src/core/lib/promise/activity.h"
 #include "src/core/lib/promise/detail/basic_join.h"
-#include "src/core/lib/promise/detail/basic_seq.h"
 #include "src/core/lib/promise/for_each.h"
 #include "src/core/lib/promise/join.h"
 #include "src/core/lib/promise/map.h"
diff --git a/test/core/promise/promise_fuzzer.cc b/test/core/promise/promise_fuzzer.cc
index cd60e1a6a9f..bf20d3f4717 100644
--- a/test/core/promise/promise_fuzzer.cc
+++ b/test/core/promise/promise_fuzzer.cc
@@ -12,6 +12,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+#include <stdint.h>
+
 #include <functional>
 #include <map>
 #include <memory>
diff --git a/test/core/promise/seq_test.cc b/test/core/promise/seq_test.cc
index 326329523e6..7e5bd9695b8 100644
--- a/test/core/promise/seq_test.cc
+++ b/test/core/promise/seq_test.cc
@@ -14,6 +14,8 @@
 
 #include "src/core/lib/promise/seq.h"
 
+#include <stdint.h>
+
 #include <memory>
 #include <string>
 #include <vector>
diff --git a/test/core/promise/try_seq_metadata_test.cc b/test/core/promise/try_seq_metadata_test.cc
index dc0fb8ca92e..2729e9fe67b 100644
--- a/test/core/promise/try_seq_metadata_test.cc
+++ b/test/core/promise/try_seq_metadata_test.cc
@@ -12,6 +12,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+#include <stdint.h>
+
 #include <memory>
 
 #include "gtest/gtest.h"
diff --git a/test/core/promise/try_seq_test.cc b/test/core/promise/try_seq_test.cc
index 126588ff08c..c45e9a20f58 100644
--- a/test/core/promise/try_seq_test.cc
+++ b/test/core/promise/try_seq_test.cc
@@ -14,6 +14,7 @@
 
 #include "src/core/lib/promise/try_seq.h"
 
+#include <stdint.h>
 #include <stdlib.h>
 
 #include <algorithm>
diff --git a/tools/codegen/core/gen_seq.py b/tools/codegen/core/gen_seq.py
new file mode 100755
index 00000000000..555f1202fa0
--- /dev/null
+++ b/tools/codegen/core/gen_seq.py
@@ -0,0 +1,271 @@
+#!/usr/bin/env python3
+
+# Copyright 2023 gRPC authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import sys
+
+from mako.template import Template
+
+seq_state = Template(
+    """
+<%def name="decl(promise_name, i, n)">
+using Promise${i} = ${promise_name};
+% if i < n-1:
+using NextFactory${i} = OncePromiseFactory<typename Promise${i}::Result, F${i}>;
+${decl(f"typename NextFactory{i}::Promise", i+1, n)}
+% endif
+</%def>
+
+<%def name="state(i, n)">
+% if i == 0:
+Promise0 current_promise;
+NextFactory0 next_factory;
+% elif i == n-1:
+union {
+    struct { ${state(i-1, n)} } prior;
+    Promise${i} current_promise;
+};
+% else:
+union {
+    struct { ${state(i-1, n)} } prior;
+    P${i} current_promise;
+};
+NextFactory${i} next_factory;
+% endif
+</%def>
+
+template <template<typename> class Traits, typename P, ${",".join(f"typename F{i}" for i in range(0,n-1))}>
+struct SeqState<Traits, P, ${",".join(f"F{i}" for i in range(0,n-1))}> {
+<% name="PromiseLike<P>" %>
+% for i in range(0,n-1):
+using Promise${i} = ${name};
+using PromiseResult${i} = typename Promise${i}::Result;
+using PromiseResultTraits${i} = Traits<PromiseResult${i}>;
+using NextFactory${i} = OncePromiseFactory<typename PromiseResultTraits${i}::UnwrappedType, F${i}>;
+<% name=f"typename NextFactory{i}::Promise" %>\\
+% endfor
+using Promise${n-1} = ${name};
+using PromiseResult${n-1} = typename Promise${n-1}::Result;
+using PromiseResultTraits${n-1} = Traits<PromiseResult${n-1}>;
+using Result = typename PromiseResultTraits${n-1}::WrappedType;
+% if n == 1:
+Promise0 current_promise;
+% else:
+%  for i in range(0,n-1):
+struct Running${i} {
+%   if i != 0:
+union {
+  GPR_NO_UNIQUE_ADDRESS Running${i-1} prior;
+%   endif
+  GPR_NO_UNIQUE_ADDRESS Promise${i} current_promise;
+%   if i != 0:
+};
+%   endif
+GPR_NO_UNIQUE_ADDRESS NextFactory${i} next_factory;
+};
+%  endfor
+union {
+    GPR_NO_UNIQUE_ADDRESS Running${n-2} prior;
+    GPR_NO_UNIQUE_ADDRESS Promise${n-1} current_promise;
+};
+% endif
+  enum class State : uint8_t { ${",".join(f"kState{i}" for i in range(0,n))} };
+  GPR_NO_UNIQUE_ADDRESS State state = State::kState0;
+
+  SeqState(P&& p, ${",".join(f"F{i}&& f{i}" for i in range(0,n-1))}) noexcept {
+    Construct(&${"prior."*(n-1)}current_promise, std::forward<P>(p));
+% for i in range(0,n-1):
+    Construct(&${"prior."*(n-1-i)}next_factory, std::forward<F${i}>(f${i}));
+% endfor
+  }
+  ~SeqState() {
+    switch (state) {
+% for i in range(0,n-1):
+     case State::kState${i}:
+      Destruct(&${"prior."*(n-1-i)}current_promise);
+      goto tail${i};
+% endfor
+     case State::kState${n-1}:
+      Destruct(&current_promise);
+      return;
+    }
+% for i in range(0,n-1):
+tail${i}:
+    Destruct(&${"prior."*(n-1-i)}next_factory);
+% endfor
+  }
+  SeqState(const SeqState& other) noexcept : state(other.state) {
+    GPR_ASSERT(state == State::kState0);
+    Construct(&${"prior."*(n-1-i)}current_promise,
+            other.${"prior."*(n-1-i)}current_promise);
+% for i in range(0,n-1):
+    Construct(&${"prior."*(n-1-i)}next_factory,
+              other.${"prior."*(n-1-i)}next_factory);
+% endfor
+  }
+  SeqState& operator=(const SeqState& other) = delete;
+  SeqState(SeqState&& other) noexcept : state(other.state) {
+    switch (state) {
+% for i in range(0,n-1):
+     case State::kState${i}:
+      Construct(&${"prior."*(n-1-i)}current_promise,
+                std::move(other.${"prior."*(n-1-i)}current_promise));
+      goto tail${i};
+% endfor
+     case State::kState${n-1}:
+      Construct(&current_promise, std::move(other.current_promise));
+      return;
+    }
+% for i in range(0,n-1):
+tail${i}:
+    Construct(&${"prior."*(n-1-i)}next_factory,
+              std::move(other.${"prior."*(n-1-i)}next_factory));
+% endfor
+  }
+  SeqState& operator=(SeqState&& other) = delete;
+  Poll<Result> PollOnce() {
+    switch (state) {
+% for i in range(0,n-1):
+      case State::kState${i}: {
+        auto result = ${"prior."*(n-1-i)}current_promise();
+        PromiseResult${i}* p = result.value_if_ready();
+        if (p == nullptr) return Pending{};
+        if (!PromiseResultTraits${i}::IsOk(*p)) {
+            return PromiseResultTraits${i}::template ReturnValue<Result>(std::move(*p));
+        }
+        Destruct(&${"prior."*(n-1-i)}current_promise);
+        auto next_promise = PromiseResultTraits${i}::CallFactory(&${"prior."*(n-1-i)}next_factory, std::move(*p));
+        Destruct(&${"prior."*(n-1-i)}next_factory);
+        Construct(&${"prior."*(n-2-i)}current_promise, std::move(next_promise));
+        state = State::kState${i+1};
+      }
+      ABSL_FALLTHROUGH_INTENDED;
+% endfor
+      default:
+      case State::kState${n-1}: {
+        auto result = current_promise();
+        auto* p = result.value_if_ready();
+        if (p == nullptr) return Pending{};
+        return Result(std::move(*p));
+      }
+    }
+  }
+};"""
+)
+
+front_matter = """
+#ifndef GRPC_SRC_CORE_LIB_PROMISE_DETAIL_SEQ_STATE_H
+#define GRPC_SRC_CORE_LIB_PROMISE_DETAIL_SEQ_STATE_H
+
+// This file is generated by tools/codegen/core/gen_seq.py
+
+#include <grpc/support/port_platform.h>
+
+#include <stdint.h>
+
+#include <utility>
+
+#include "absl/base/attributes.h"
+
+#include <grpc/support/log.h>
+
+#include "src/core/lib/gprpp/construct_destruct.h"
+#include "src/core/lib/promise/detail/promise_factory.h"
+#include "src/core/lib/promise/detail/promise_like.h"
+#include "src/core/lib/promise/poll.h"
+
+// A sequence under some traits for some set of callables P, Fs.
+// P should be a promise-like object that yields a value.
+// Fs... should be promise-factory-like objects that take the value from the
+// previous step and yield a promise. Note that most of the machinery in
+// PromiseFactory exists to make it possible for those promise-factory-like
+// objects to be anything that's convenient.
+// Traits defines how we move from one step to the next. Traits sets up the 
+// wrapping and escape handling for the sequence. 
+// Promises return wrapped values that the trait can inspect and unwrap before
+// passing them to the next element of the sequence. The trait can
+// also interpret a wrapped value as an escape value, which terminates
+// evaluation of the sequence immediately yielding a result. Traits for type T
+// have the members:
+//  * type UnwrappedType - the type after removing wrapping from T (i.e. for
+//    TrySeq, T=StatusOr<U> yields UnwrappedType=U).
+//  * type WrappedType - the type after adding wrapping if it doesn't already
+//    exist (i.e. for TrySeq if T is not Status/StatusOr/void, then
+//    WrappedType=StatusOr<T>; if T is Status then WrappedType=Status (it's
+//    already wrapped!))
+//  * template <typename Next> void CallFactory(Next* next_factory, T&& value) -
+//    call promise factory next_factory with the result of unwrapping value, and
+//    return the resulting promise.
+//  * template <typename Result, typename RunNext> Poll<Result>
+//    CheckResultAndRunNext(T prior, RunNext run_next) - examine the value of
+//    prior, and decide to escape or continue. If escaping, return the final
+//    sequence value of type Poll<Result>. If continuing, return the value of
+//    run_next(std::move(prior)).
+//
+// A state contains the current promise, and the promise factory to turn the
+// result of the current promise into the next state's promise. We play a shell
+// game such that the prior state and our current promise are kept in a union,
+// and the next promise factory is kept alongside in the state struct.
+// Recursively this guarantees that the next functions get initialized once, and
+// destroyed once, and don't need to be moved around in between, which avoids a
+// potential O(n**2) loop of next factory moves had we used a variant of states
+// here. The very first state does not have a prior state, and so that state has
+// a partial specialization below. The final state does not have a next state;
+// that state is inlined in BasicSeq since that was simpler to type.
+
+namespace grpc_core {
+namespace promise_detail {
+template <template<typename> class Traits, typename P, typename... Fs>
+struct SeqState;
+"""
+
+end_matter = """
+}  // namespace promise_detail
+}  // namespace grpc_core
+
+#endif  // GRPC_SRC_CORE_LIB_PROMISE_DETAIL_SEQ_STATE_H
+"""
+
+
+# utility: print a big comment block into a set of files
+def put_banner(files, banner):
+    for f in files:
+        for line in banner:
+            print("// %s" % line, file=f)
+        print("", file=f)
+
+
+with open(sys.argv[0]) as my_source:
+    copyright = []
+    for line in my_source:
+        if line[0] != "#":
+            break
+    for line in my_source:
+        if line[0] == "#":
+            copyright.append(line)
+            break
+    for line in my_source:
+        if line[0] != "#":
+            break
+        copyright.append(line)
+
+copyright = [line[2:].rstrip() for line in copyright]
+
+with open("src/core/lib/promise/detail/seq_state.h", "w") as f:
+    put_banner([f], copyright)
+    print(front_matter, file=f)
+    for n in range(2, 10):
+        print(seq_state.render(n=n), file=f)
+    print(end_matter, file=f)
diff --git a/tools/doxygen/Doxyfile.c++.internal b/tools/doxygen/Doxyfile.c++.internal
index 637de7d2295..852a157e368 100644
--- a/tools/doxygen/Doxyfile.c++.internal
+++ b/tools/doxygen/Doxyfile.c++.internal
@@ -2450,8 +2450,8 @@ src/core/lib/promise/context.h \
 src/core/lib/promise/detail/basic_seq.h \
 src/core/lib/promise/detail/promise_factory.h \
 src/core/lib/promise/detail/promise_like.h \
+src/core/lib/promise/detail/seq_state.h \
 src/core/lib/promise/detail/status.h \
-src/core/lib/promise/detail/switch.h \
 src/core/lib/promise/exec_ctx_wakeup_scheduler.h \
 src/core/lib/promise/for_each.h \
 src/core/lib/promise/if.h \
diff --git a/tools/doxygen/Doxyfile.core.internal b/tools/doxygen/Doxyfile.core.internal
index 17607e9339a..a61cf0a9f98 100644
--- a/tools/doxygen/Doxyfile.core.internal
+++ b/tools/doxygen/Doxyfile.core.internal
@@ -2231,8 +2231,8 @@ src/core/lib/promise/context.h \
 src/core/lib/promise/detail/basic_seq.h \
 src/core/lib/promise/detail/promise_factory.h \
 src/core/lib/promise/detail/promise_like.h \
+src/core/lib/promise/detail/seq_state.h \
 src/core/lib/promise/detail/status.h \
-src/core/lib/promise/detail/switch.h \
 src/core/lib/promise/exec_ctx_wakeup_scheduler.h \
 src/core/lib/promise/for_each.h \
 src/core/lib/promise/if.h \