Stop wrapping __has_include in another macro

[arichardson]

Summary

This is not guaranteed to work since the characters after 'has_include('
have special lexing rules that can't possibly be applied when
has_include is generated by a macro. It also breaks the crash reproducers
generated by -frewrite-includes (see https://llvm.org/pr37990).

[arichardson]

Ping?

[EricWF]

Where are the special lexing rules specified?

include/__config

153

I do prefer not hijacking this name, but if it's needed to make things work, then it's OK with me.

@mclow.lists Are you OK if we steal this identifier and #define it ourselves.

[arichardson]

In D49067#1172500, @EricWF wrote:

Where are the special lexing rules specified?

In https://bugs.llvm.org/show_bug.cgi?id=37990#c1 @rsmith wrote:

The C++ standard appears to be missing a rule that says that a __has_include token produced by macro expansion results in undefined behavior (matching the corresponding rule for a 'defined' token produced by macro expansion), but that seems to just be a wording oversight. You should not expect this to work.

Looks like the special rules are explained in 3.2.2 of https://isocpp.org/std/standing-documents/sd-6-sg10-feature-test-recommendations#recs.hasinc

In the first form of the has-include-expression, the parenthesized header-name token is not subject to macro expansion. The second and third forms are considered only if the first form does not match, and the preprocessing tokens are processed just as in normal text.

While there is no mention of the define to 0 if not present for __has_include it is used in the example for __has_cpp_attribute in https://isocpp.org/std/standing-documents/sd-6-sg10-feature-test-recommendations#recs.hasattr.

include/__config

153

This is the correct way of dealing with missing __has_include() according to @rsmith

[rsmith]

include/__config

153

If you would prefer to not define this name yourself, the other option would be to replace all uses of __has_include(blah) with

#ifdef __has_include
#if __has_include(blah)
//...
#endif
#endif

1062–1075

Example:

#if defined(__MINGW32__) && defined(__has_include)
#if __has_include(<pthread.h>)
#define _LIBCPP_MINGW32_HAS_PTHREAD
#endif
#endif
#if defined(__FreeBSD__) || \
// [...]
    defined(__sun__) || \
    defined(_LIBCPP_MINGW32_HAS_PTHREAD)

[mclow.lists]

include/__config

153

Agreed with @EricWF in general - we prefer to wrap rather than hijack other people's names.

However, we can't wrap here, so I'm ok with the hijack.

include/__config

 #endif
 
 #ifndef __has_declspec_attribute
 #define __has_declspec_attribute(__x) 0
 #endif
 
 #define __has_keyword(__x) !(__is_identifier(__x))
 
-#ifdef __has_include
-#  define __libcpp_has_include(__x) __has_include(__x)
-#else
-#  define __libcpp_has_include(__x) 0
+#ifndef __has_include
+#define __has_include(...) 0
 #endif
 
 #if defined(__clang__)
 #  define _LIBCPP_COMPILER_CLANG
 #  ifndef __apple_build_version__
 #    define _LIBCPP_CLANG_VER (__clang_major__ * 100 + __clang_minor__)
 #  endif
 #elif defined(__GNUC__)

 #define _LIBCPP_WEAK __attribute__((__weak__))
 #endif
 
 // Thread API
 #if !defined(_LIBCPP_HAS_NO_THREADS) && \
     !defined(_LIBCPP_HAS_THREAD_API_PTHREAD) && \
     !defined(_LIBCPP_HAS_THREAD_API_WIN32) && \
     !defined(_LIBCPP_HAS_THREAD_API_EXTERNAL)
 #  if defined(__FreeBSD__) || \
       defined(__Fuchsia__) || \
       defined(__NetBSD__) || \
       defined(__linux__) || \
       defined(__APPLE__) || \
       defined(__CloudABI__) || \
       defined(__sun__) || \
-      (defined(__MINGW32__) && __libcpp_has_include(<pthread.h>))
+      (defined(__MINGW32__) && __has_include(<pthread.h>))
 #    define _LIBCPP_HAS_THREAD_API_PTHREAD
 #  elif defined(_LIBCPP_WIN32API)
 #    define _LIBCPP_HAS_THREAD_API_WIN32
 #  else
 #    error "No thread API"
 #  endif // _LIBCPP_HAS_THREAD_API
 #endif // _LIBCPP_HAS_NO_THREADS
 
 #if defined(_LIBCPP_HAS_NO_THREADS) && defined(_LIBCPP_HAS_THREAD_API_PTHREAD)
 #error _LIBCPP_HAS_THREAD_API_PTHREAD may only be defined when \
        _LIBCPP_HAS_NO_THREADS is not defined.
 #endif
 
 #if defined(_LIBCPP_HAS_NO_THREADS) && defined(_LIBCPP_HAS_THREAD_API_EXTERNAL)

test/support/test_macros.h

 #define TEST_CONCAT(X, Y) TEST_CONCAT1(X, Y)
 
 #ifdef __has_feature
 #define TEST_HAS_FEATURE(X) __has_feature(X)
 #else
 #define TEST_HAS_FEATURE(X) 0
 #endif
 
-#ifdef __has_include
-#define TEST_HAS_INCLUDE(X) __has_include(X)
-#else
-#define TEST_HAS_INCLUDE(X) 0
+#ifndef __has_include
+#define __has_include(...) 0
 #endif
 
 #ifdef __has_extension
 #define TEST_HAS_EXTENSION(X) __has_extension(X)
 #else
 #define TEST_HAS_EXTENSION(X) 0
 #endif
 

 # define TEST_STD_VER 17
 #else
 # define TEST_STD_VER 99    // greater than current standard
 // This is deliberately different than _LIBCPP_STD_VER to discourage matching them up.
 #endif
 #endif
 
 // Attempt to deduce GCC version
-#if defined(_LIBCPP_VERSION) && TEST_HAS_INCLUDE(<features.h>)
+#if defined(_LIBCPP_VERSION) && __has_include(<features.h>)
 #include <features.h>
 #define TEST_HAS_GLIBC
 #define TEST_GLIBC_PREREQ(major, minor) __GLIBC_PREREQ(major, minor)
 #endif
 
 /* Features that were introduced in C++14 */
 #if TEST_STD_VER >= 14
 #define TEST_HAS_EXTENDED_CONSTEXPR