[libcxx-commits] [PATCH] D150015: [libc++] Consistently enable __CORRECT_ISO_CPP_WCHAR_H_PROTO in mbstate.

[Jordan Rupprecht via Phabricator via libcxx-commits]

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In libc++'s `wchar.h`, before we forward to the system `wchar.h`, we set `__CORRECT_ISO_CPP_WCHAR_H_PROTO` to ensure it defines the correct signature (e.g. `extern "C++" const wchar_t *wmemchr` and not `extern wchar_t *wmemchr`). After D148542 <https://reviews.llvm.org/D148542>, there are cases where we include the system `wchar.h` from within `__mbstate_t.h` without setting that, and so we get a function type mismatch if we transitively include `wchar.h` through multiple headers in a modules-enabled build. Consistently setting it here resolves those build errors.

Alternative 1: we could put this in `__config` instead. I chose to put it here for a more limited scope.

Alternative 2: we could patch `wchar.h` itself to work correctly and remove references `__CORRECT_ISO_CPP_WCHAR_H_PROTO` from libc++ entirely. It does already set it, but with an additional condition that it is being built by GCC >= 4.4. Clang does pretend to be GCC via `__GNUC__` etc. which can be controlled via `-fgnuc-version` command line flags, but that might have other consequences.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D150015

Files:
  libcxx/include/__mbstate_t.h


Index: libcxx/include/__mbstate_t.h
===================================================================
--- libcxx/include/__mbstate_t.h
+++ libcxx/include/__mbstate_t.h
@@ -27,6 +27,12 @@
 // This does not define std::mbstate_t -- this only brings in the declaration
 // in the global namespace.
 
+// Consistently set this the same as in libc++'s <wchar.h> header to avoid
+// different function signatures.
+#ifdef __cplusplus
+#  define __CORRECT_ISO_CPP_WCHAR_H_PROTO
+#endif
+
 #if __has_include(<bits/types/mbstate_t.h>)
 #   include <bits/types/mbstate_t.h> // works on most Unixes
 #elif __has_include(<sys/_types/_mbstate_t.h>)


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