Avoid a empty library build failure on Apple platforms

https://github.com/abseil/abseil-cpp/issues/1465 reports that
some CMake builds on Apply platforms issue

```
warning: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/ranlib: archive library: libabsl_bad_any_cast_impl.a the table of contents is empty (no object file members in the library define global symbols)
```

Our CMake build handles this problem for header-only libraries by not
building a library at all. For some libraries, for example our polyfills,
the library is only conditionally empty. In these libraries, I added
a single char variable on Apple platforms as a workaround.

I have been able to reproduce the warnings reported in
https://github.com/abseil/abseil-cpp/issues/1465, but they don't fail the
build for me. I don't see them any more after this change.

PiperOrigin-RevId: 595480705
Change-Id: Ie48637e84ebae2f2aea4e2de83b146f30f6a76b9

3 files changed, 54 insertions, 0 deletions

diff --git a/absl/types/bad_any_cast.cc b/absl/types/bad_any_cast.cc
index b0592cc9..22558b48 100644
--- a/absl/types/bad_any_cast.cc
+++ b/absl/types/bad_any_cast.cc
@@ -43,4 +43,22 @@ void ThrowBadAnyCast() {
 ABSL_NAMESPACE_END
 }  // namespace absl
 
+#else
+
+// https://github.com/abseil/abseil-cpp/issues/1465
+// CMake builds on Apple platforms error when libraries are empty.
+// Our CMake configuration can avoid this error on header-only libraries,
+// but since this library is conditionally empty, including a single
+// variable is an easy workaround.
+#ifdef __APPLE__
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace types_internal {
+extern const char kAvoidEmptyBadAnyCastLibraryWarning;
+const char kAvoidEmptyBadAnyCastLibraryWarning = 0;
+}  // namespace types_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+#endif  // __APPLE__
+
 #endif  // ABSL_USES_STD_ANY
diff --git a/absl/types/bad_optional_access.cc b/absl/types/bad_optional_access.cc
index 26aca70d..2552cc85 100644
--- a/absl/types/bad_optional_access.cc
+++ b/absl/types/bad_optional_access.cc
@@ -45,4 +45,22 @@ void throw_bad_optional_access() {
 ABSL_NAMESPACE_END
 }  // namespace absl
 
+#else
+
+// https://github.com/abseil/abseil-cpp/issues/1465
+// CMake builds on Apple platforms error when libraries are empty.
+// Our CMake configuration can avoid this error on header-only libraries,
+// but since this library is conditionally empty, including a single
+// variable is an easy workaround.
+#ifdef __APPLE__
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace types_internal {
+extern const char kAvoidEmptyBadOptionalAccessLibraryWarning;
+const char kAvoidEmptyBadOptionalAccessLibraryWarning = 0;
+}  // namespace types_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+#endif  // __APPLE__
+
 #endif  // ABSL_USES_STD_OPTIONAL
diff --git a/absl/types/bad_variant_access.cc b/absl/types/bad_variant_access.cc
index 3dc88cc0..a76aa80d 100644
--- a/absl/types/bad_variant_access.cc
+++ b/absl/types/bad_variant_access.cc
@@ -61,4 +61,22 @@ void Rethrow() {
 ABSL_NAMESPACE_END
 }  // namespace absl
 
+#else
+
+// https://github.com/abseil/abseil-cpp/issues/1465
+// CMake builds on Apple platforms error when libraries are empty.
+// Our CMake configuration can avoid this error on header-only libraries,
+// but since this library is conditionally empty, including a single
+// variable is an easy workaround.
+#ifdef __APPLE__
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace types_internal {
+extern const char kAvoidEmptyBadVariantAccessLibraryWarning;
+const char kAvoidEmptyBadVariantAccessLibraryWarning = 0;
+}  // namespace types_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+#endif  // __APPLE__
+
 #endif  // ABSL_USES_STD_VARIANT