Turn on -Wdeprecated-enum-enum-conversion in protobuf code, and fix existing warnings.

PiperOrigin-RevId: 497985362

src/google/protobuf/compiler/cpp/parse_function_generator.cc

@@ -708,7 +708,9 @@ static void FormatFieldKind(Formatter& format,
   const int rep_index = (type_card & fl::kRepMask) >> fl::kRepShift;
   const int tv_index = (type_card & fl::kTvMask) >> fl::kTvShift;
 
-  format("(");
+  // Use `0|` prefix to eagerly convert the enums to int to avoid enum-enum
+  // operations. They are deprecated in C++20.
+  format("(0 | ");
   static constexpr const char* kFieldCardNames[] = {"Singular", "Optional",
                                                     "Repeated", "Oneof"};
   static_assert((fl::kFcSingular >> fl::kFcShift) == 0, "");

src/google/protobuf/port_def.inc

@@ -971,6 +971,9 @@ static_assert(PROTOBUF_CPLUSPLUS_MIN(201402L), "Protobuf only supports C++14 and
 //   char* p = strchr(s, '\n');
 //   return p ? p - s : -1;
 #pragma clang diagnostic ignored "-Wshorten-64-to-32"
+// Turn on -Wdeprecated-enum-enum-conversion. This deprecation comes in C++20
+// via http://wg21.link/p1120r0.
+#pragma clang diagnostic error "-Wdeprecated-enum-enum-conversion"
 #endif
 #ifdef __GNUC__
 #pragma GCC diagnostic push