[libcxx] Do not include the C math.h header before __config

[miyuki]

Summary

Certain C libraries require configuration macros defined in config
to provide the correct functionality for libc++. This patch ensures
that the C header math.h is always included after the config
header. It also adds a Windows-specific #if guard for the case when
the C math.h file is included the second time, as suggested by
Marshall in https://reviews.llvm.org/rL323490.

Fixes PR36382.

[mclow.lists]

This LGTM.

[pcc]

include/math.h

1499

Nit: you don't actually need the && defined(_USE_MATH_DEFINES) part.

[mclow.lists]

include/math.h

1499

I proposed that deliberately.

With the test, if you don't define _USE_MATH_DEFINES and re-include <math.h> it does nothing; which is what you want.

The MS-supplied <math.h> probably does this already, but ...

include/math.h

 // -*- C++ -*-
 //===---------------------------- math.h ----------------------------------===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is dual licensed under the MIT and the University of Illinois Open
 // Source Licenses. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 
-// This include lives outside the header guard in order to support an MSVC
-// extension which allows users to do:
-//
-// #define _USE_MATH_DEFINES
-// #include <math.h>
-//
-// and receive the definitions of mathematical constants, even if <math.h>
-// has previously been included.
-#include_next <math.h>
-
 #ifndef _LIBCPP_MATH_H
 #define _LIBCPP_MATH_H
 
 /*
     math.h synopsis
 
 Macros:
 

 */
 
 #include <__config>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
 #endif
 
+#include_next <math.h>
+
 #ifdef __cplusplus
 
 // We support including .h headers inside 'extern "C"' contexts, so switch
 // back to C++ linkage before including these C++ headers.
 extern "C++" {
 
 #include <type_traits>
 #include <limits>

 inline _LIBCPP_INLINE_VISIBILITY
 typename std::enable_if<std::is_integral<_A1>::value, double>::type
 trunc(_A1 __lcpp_x) _NOEXCEPT {return ::trunc((double)__lcpp_x);}
 
 } // extern "C++"
 
 #endif // __cplusplus
 
+#else // _LIBCPP_MATH_H
+
+// This include lives outside the header guard in order to support an MSVC
+// extension which allows users to do:
+//
+// #define _USE_MATH_DEFINES
+// #include <math.h>
+//
+// and receive the definitions of mathematical constants, even if <math.h>
+// has previously been included.
+#if defined(_LIBCPP_MSVCRT) && defined(_USE_MATH_DEFINES)
+#include_next <math.h>
+#endif
+
 #endif  // _LIBCPP_MATH_H