[libc++] Fix disabling of extension warnings in C++20 and later

[philnik777]

__has_feature(modules) is always true in C++20 and later. Instead of using that, just disable extension warnings if they're not ignored through the system header machinery anyways.

[llvmbot]

@llvm/pr-subscribers-libcxx

Author: Nikolas Klauser (philnik777)

Changes

__has_feature(modules) is always true in C++20 and later. Instead of using that, just disable extension warnings if they're not ignored through the system header machinery anyways.

---

Full diff: https://github.com/llvm/llvm-project/pull/134989.diff

1 Files Affected:

• (modified) libcxx/include/__config (+3-3)

diff --git a/libcxx/include/__config b/libcxx/include/__config
index 084e1d4402e6c..05ae32f01e865 100644
--- a/libcxx/include/__config
+++ b/libcxx/include/__config
@@ -543,9 +543,9 @@ typedef __char32_t char32_t;
 #  endif
 
 // Clang modules take a significant compile time hit when pushing and popping diagnostics.
-// Since all the headers are marked as system headers in the modulemap, we can simply disable this
-// pushing and popping when building with clang modules.
-#  if !__has_feature(modules)
+// Since all the headers are marked as system headers unless _LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER is defined, we can
+// simply disable this pushing and popping when _LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER isn't defined.
+#  ifdef _LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER
 #    define _LIBCPP_PUSH_EXTENSION_DIAGNOSTICS                                                                         \
       _LIBCPP_DIAGNOSTIC_PUSH                                                                                          \
       _LIBCPP_CLANG_DIAGNOSTIC_IGNORED("-Wc++11-extensions")                                                           \