Disable constinit on MSVC for static library builds

38927bf cleaned up the constinit suppressions based on our understanding of the Windows/MSVC limitations at the time. It turns out this was incomplete and, by restoring constinit on static library MSVC builds, we broke something.

On MSVC, but not clang-cl, the workaround must extend to static library builds too. Although pointers to same-dll symbols are fine and MSVC, with optimizations, will avoid a static initializer, it doesn't seem to consistently recognize it in the type systems, so we can't enforce constinit. See https://godbolt.org/z/cYeYaaWq5

On MSVC, but not clang-cl, the above workaround must extend to static library builds too. MSVC can avoid a global constructor when initializing structures containing pointers to same-dll symbols, it relies on the optimizer for this, so we can't enforce constinit. This limitation does not apply to constexpr. See https://godbolt.org/z/hsT9e3zs4

Add a Windows Bazel C++20 build configuration, which would have caught this.

PiperOrigin-RevId: 858801389

Author: protobuf-github-bot

.github/workflows/test_cpp.yml

@@ -431,6 +431,10 @@ jobs:
             os: windows-2022
             cache_key: windows-2022-msvc-cl
             bazel: test //src/...  --config=msvc-cl --test_tag_filters=-conformance --build_tag_filters=-conformance
+          - name: Windows Bazel C++20
+            os: windows-2022
+            cache_key: windows-2022-msvc-cl
+            bazel: test //src/...  --config=msvc-cl --cxxopt="-std:c++20" --test_tag_filters=-conformance --build_tag_filters=-conformance
           - name: Windows Bazel clang-cl
             os: windows-2022
             cache_key: windows-2022-clang-cl

src/google/protobuf/port_def.inc

@@ -424,6 +424,13 @@ static_assert(PROTOBUF_ABSL_MIN(20230125, 3),
 // https://github.com/protocolbuffers/protobuf/pull/13240. Work around this by
 // suppressing `constinit`.
 #define PROTOBUF_CONSTINIT
+#elif defined(_MSC_VER) && !defined(__clang__)
+// On MSVC, but not clang-cl, the above workaround must extend to static library
+// builds too. MSVC can avoid a global constructor when initializing structures
+// containing pointers to same-dll symbols, it relies on the optimizer for this,
+// so we can't enforce constinit. This limitation does not apply to constexpr.
+// See https://godbolt.org/z/hsT9e3zs4
+#define PROTOBUF_CONSTINIT
 #elif defined(__GNUC__) && !defined(__clang__)
 // GCC doesn't support constinit aggregate initialization of absl::Cord.
 #define PROTOBUF_CONSTINIT