[libcxx-commits] [libcxx] [libc++] Fix C++23 standard modules when using with `clang-cl` on Windows (PR #148992)

[A. Jiang via libcxx-commits]

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@@ -51,6 +51,9 @@ module;
 #include <cstdio>
 #include <cstdlib>
 #include <cstring>
+#ifdef _LIBCPP_ABI_VCRUNTIME
+#define _BUILD_STD_MODULE
+#endif
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frederick-vs-ja wrote:

It's quite weird to define the macro here. Can we define it before `#include <__config>` or even `module;`? Also, I think we should change `generate_libcxx_cppm_in.py` to generate the macro definition.

https://github.com/llvm/llvm-project/pull/148992