Port C++20 Requires to it's C++17 equivalent for reuse across protos

PiperOrigin-RevId: 635894605

protos/BUILD

@@ -167,3 +167,9 @@ cc_test(
         "@com_google_googletest//:gtest_main",
     ],
 )
+
+cc_library(
+    name = "requires",
+    hdrs = ["requires.h"],
+    visibility = ["//visibility:public"],
+)

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_

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",
     ],

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); }