[PATCH] D31725: [libc++] Add _LIBCPP_DISABLE_EXTERN_TEMPLATE config option

[Shoaib Meenai via Phabricator via cfe-commits]

smeenai created this revision.

When the libc++ extern template macros were added, the intent was for it
to be possible for consumers of the headers to disable extern templates
(via `-D_LIBCPP_EXTERN_TEMPLATE(...)=`). Unfortunately, support for
specifying function-like macros varies on the command line varies across
compilers (e.g. MSVC doesn't support it at all), and cmake doesn't allow
it for the same reason. Add a non-function macro for this purpose.

The intended use is for libraries which want to use the libc++ headers
without taking a dependency on the libc++ library itself. I can name the
macro something which reflects its intent rather than its behavior (e.g.
`_LIBCPP_HEADER_ONLY`) if desired.


https://reviews.llvm.org/D31725

Files:
  include/__config


Index: include/__config
===================================================================
--- include/__config
+++ include/__config
@@ -837,6 +837,11 @@
 # endif
 #endif
 
+#ifdef _LIBCPP_DISABLE_EXTERN_TEMPLATE
+#define _LIBCPP_EXTERN_TEMPLATE(...)
+#define _LIBCPP_EXTERN_TEMPLATE2(...)
+#endif
+
 #ifndef _LIBCPP_EXTERN_TEMPLATE
 #define _LIBCPP_EXTERN_TEMPLATE(...) extern template __VA_ARGS__;
 #endif


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