Add a compile test for headers that require C compatibility

windows_clangcl_bazel.bat includes a change from --copt to --cxxopt to
only pass /std:c++XX to C++ compiles

PiperOrigin-RevId: 646157298
Change-Id: Ib6d9861a2b2d45eb0d664f23b6f3a7426f8e0ab3

absl/base/BUILD.bazel

@@ -340,6 +340,18 @@ cc_test(
     ],
 )
 
+cc_test(
+    name = "c_header_test",
+    srcs = ["c_header_test.c"],
+    tags = [
+        "no_test_wasm",
+    ],
+    deps = [
+        ":config",
+        ":core_headers",
+    ],
+)
+
 cc_library(
     name = "throw_delegate",
     srcs = ["internal/throw_delegate.cc"],

absl/base/c_header_test.c

+// Copyright 2024 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifdef __cplusplus
+#error This is a C compile test
+#endif
+
+// This test ensures that headers that are included in legacy C code are
+// compatible with C. Abseil is a C++ library. We do not desire to expand C
+// compatibility or keep C compatibility forever. This test only exists to
+// ensure C compatibility until it is no longer required. Do not add new code
+// that requires C compatibility.
+#include "absl/base/attributes.h"     // IWYU pragma: keep
+#include "absl/base/config.h"         // IWYU pragma: keep
+#include "absl/base/optimization.h"   // IWYU pragma: keep
+#include "absl/base/policy_checks.h"  // IWYU pragma: keep
+#include "absl/base/port.h"           // IWYU pragma: keep
+
+int main() { return 0; }

ci/windows_clangcl_bazel.bat

@@ -43,9 +43,9 @@ IF NOT "%ALTERNATE_OPTIONS%"=="" copy %ALTERNATE_OPTIONS% absl\base\options.h
   test ... ^
   --compilation_mode=%COMPILATION_MODE% ^
   --compiler=clang-cl ^
-  --copt=/std:%STD% ^
   --copt=/WX ^
   --copt=-Wno-microsoft-cast ^
+  --cxxopt=/std:%STD% ^
   --define=absl=1 ^
   --distdir=%KOKORO_GFILE_DIR%\distdir ^
   --enable_bzlmod=true ^