[promises] Improve seq test (#32462)

I had some doubts about `Seq` debugging another problem, so expanded the
tests we have to try and isolate the problem (so far without success, so
I think the original problem was elsewhere).
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src/core/lib/promise/detail/basic_seq.h

@@ -168,7 +168,7 @@ auto CallNext(SeqState<Traits, I, Fs...>* state, T&& arg)
       &state->next_factory, std::forward<T>(arg));
 }
 
-// A sequence under stome traits for some set of callables Fs.
+// 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

test/core/promise/seq_test.cc

@@ -14,6 +14,7 @@
 
 #include "src/core/lib/promise/seq.h"
 
+#include <memory>
 #include <string>
 #include <vector>
 
@@ -27,9 +28,21 @@ TEST(SeqTest, Immediate) {
 }
 
 TEST(SeqTest, OneThen) {
-  auto initial = [] { return 3; };
-  auto then = [](int i) { return [i]() { return i + 4; }; };
-  EXPECT_EQ(Seq(initial, then)(), Poll<int>(7));
+  auto initial = [a = std::make_unique<int>(0)] { return 3; };
+  auto then = [a = std::make_unique<int>(1)](int i) {
+    return [i, b = std::make_unique<int>(2)]() { return i + 4; };
+  };
+  EXPECT_EQ(Seq(std::move(initial), std::move(then))(), Poll<int>(7));
+}
+
+TEST(SeqTest, OneThenIncomplete) {
+  auto initial = [a = std::make_unique<int>(0)]() -> Poll<int> {
+    return Pending{};
+  };
+  auto then = [a = std::make_unique<int>(1)](int i) {
+    return [i, b = std::make_unique<int>(2)]() { return i + 4; };
+  };
+  EXPECT_EQ(Seq(std::move(initial), std::move(then))(), Poll<int>(Pending{}));
 }
 
 TEST(SeqTest, TwoTypedThens) {
@@ -60,10 +73,17 @@ TEST(SeqTest, TwoThens) {
 }
 
 TEST(SeqTest, ThreeThens) {
-  EXPECT_EQ(Seq([] { return std::string("a"); },
-                [](std::string i) { return [i]() { return i + "b"; }; },
-                [](std::string i) { return [i]() { return i + "c"; }; },
-                [](std::string i) { return [i]() { return i + "d"; }; })(),
+  EXPECT_EQ(
+      Seq([x = std::make_unique<int>(1)] { return std::string("a"); },
+          [x = std::make_unique<int>(1)](std::string i) {
+            return [i, y = std::make_unique<int>(2)]() { return i + "b"; };
+          },
+          [x = std::make_unique<int>(1)](std::string i) {
+            return [i, y = std::make_unique<int>(2)]() { return i + "c"; };
+          },
+          [x = std::make_unique<int>(1)](std::string i) {
+            return [i, y = std::make_unique<int>(2)]() { return i + "d"; };
+          })(),
       Poll<std::string>("abcd"));
 }