From b94e117faab7e0feb5c5e92b2bb7c9d1e16f2c5a Mon Sep 17 00:00:00 2001
From: Hong Shin <hongshin@google.com>
Date: Tue, 21 May 2024 12:37:42 -0700
Subject: [PATCH] Port C++20 Requires to it's C++17 equivalent for reuse across
 protos

PiperOrigin-RevId: 635894605
---
 protos/BUILD                             |  6 ++++++
 protos/requires.h                        | 17 +++++++++++++++++
 protos_generator/tests/BUILD             |  1 +
 protos_generator/tests/test_generated.cc |  8 ++------
 4 files changed, 26 insertions(+), 6 deletions(-)
 create mode 100644 protos/requires.h

diff --git a/protos/BUILD b/protos/BUILD
index 0e0032c37b..76c287e4fb 100644
--- a/protos/BUILD
+++ b/protos/BUILD
@@ -167,3 +167,9 @@ cc_test(
         "@com_google_googletest//:gtest_main",
     ],
 )
+
+cc_library(
+    name = "requires",
+    hdrs = ["requires.h"],
+    visibility = ["//visibility:public"],
+)
diff --git a/protos/requires.h b/protos/requires.h
new file mode 100644
index 0000000000..f7ea016392
--- /dev/null
+++ b/protos/requires.h
@@ -0,0 +1,17 @@
+#ifndef THIRD_PARTY_UPB_PROTOS_REQUIRES_H_
+#define THIRD_PARTY_UPB_PROTOS_REQUIRES_H_
+
+#include <type_traits>
+namespace protos::internal {
+// Ports C++20 `requires` to C++17.
+// C++20 ideal:
+//  if constexpr (requires { t.foo(); }) { ... }
+// Our C++17 stopgap solution:
+//  if constexpr (Requires<T>([](auto x) -> decltype(x.foo()) {})) { ... }
+template <typename... T, typename F>
+constexpr bool Requires(F) {
+  return std::is_invocable_v<F, T...>;
+}
+}  // namespace protos::internal
+
+#endif  // THIRD_PARTY_UPB_PROTOS_REQUIRES_H_
diff --git a/protos_generator/tests/BUILD b/protos_generator/tests/BUILD
index fabee7fe4a..98042ca09c 100644
--- a/protos_generator/tests/BUILD
+++ b/protos_generator/tests/BUILD
@@ -131,6 +131,7 @@ cc_test(
         "@com_google_absl//absl/status:statusor",
         "@com_google_absl//absl/strings",
         "//protos",
+        "//protos:requires",
         "//upb:mem",
         "//protos:repeated_field",
     ],
diff --git a/protos_generator/tests/test_generated.cc b/protos_generator/tests/test_generated.cc
index 208a1be80a..61bd66fd6e 100644
--- a/protos_generator/tests/test_generated.cc
+++ b/protos_generator/tests/test_generated.cc
@@ -21,6 +21,7 @@
 #include "protos/protos.h"
 #include "protos/repeated_field.h"
 #include "protos/repeated_field_iterator.h"
+#include "protos/requires.h"
 #include "protos_generator/tests/child_model.upb.proto.h"
 #include "protos_generator/tests/no_package.upb.proto.h"
 #include "protos_generator/tests/test_model.upb.proto.h"
@@ -29,6 +30,7 @@
 
 namespace {
 
+using ::protos::internal::Requires;
 using ::protos_generator::test::protos::ChildModel1;
 using ::protos_generator::test::protos::container_ext;
 using ::protos_generator::test::protos::ContainerExtension;
@@ -44,12 +46,6 @@ using ::protos_generator::test::protos::theme;
 using ::protos_generator::test::protos::ThemeExtension;
 using ::testing::ElementsAre;
 
-// C++17 port of C++20 `requires`
-template <typename... T, typename F>
-constexpr bool Requires(F) {
-  return std::is_invocable_v<F, T...>;
-}
-
 TEST(CppGeneratedCode, Constructor) { TestModel test_model; }
 
 TEST(CppGeneratedCode, MessageEnum) { EXPECT_EQ(5, TestModel_Category_IMAGES); }