Headers: fix collisions with .h files of other projects

[andrewrk]

Summary

stdarg.h:

• va_list - macos _va_list.h was duplicately defining va_list. Fixed by this ifndef _VA_LIST_T
• additionally define _VA_LIST_DEFINED because glibc stdio.h was duplicately defining va_list

stddef.h

• ptrdiff_t - wrap in _PTRDIFF_T_DEFINED to protect against mingw defining it twice
• size_t - protections against collisions with mingw
• wchar_t - protections against duplicate definition with mingw

All of these came up in real world scenarios when using libclang to parse .h files in the Zig project.
These are the patches that Zig has on top of libclang headers, and if this patch is merged, then
Zig project will be tracking clang trunk.

[Qix-]

lib/Headers/stdarg.h

30

Super nit-picky but you could condense this a bit by using

#if !defined(_VA_LIST) && !defined(_VA_LIST_T)

and a single #endif (revert the addition of line 34).

It's arguably easier to understand intent instead of adding another level of nesting. Same thing goes for the other two sections.

Just a suggestion.

[andrewrk]

Addressed review feedback

[efriedma]

What is this change actually solving? Even if the typedef is actually redundant, it shouldn't cause a compiler error: redundant typedefs are allowed in the latest versions of the C and C++ standards. (And even if you were targeting an earlier standard, we normally suppress warnings in system headers.)

[andrewrk]

In D51265#1215060, @efriedma wrote:

What is this change actually solving? Even if the typedef is actually redundant, it shouldn't cause a compiler error: redundant typedefs are allowed in the latest versions of the C and C++ standards. (And even if you were targeting an earlier standard, we normally suppress warnings in system headers.)

Here's an example use case that this solves:

• Install MXE (See https://mxe.cc/)
• Install the png package
• Use libclang API to parse png.h
• Expect that there are no errors, but there are errors

[efriedma]

Can you give the text of the error messages?

If you're seeing a "typedef redefinition with different types" error, libclang is getting confused about the target. And parsing code for the wrong target cannot work in general.

lib/Headers/stdarg.h

 #ifndef __STDARG_H
 #define __STDARG_H
 
-#ifndef _VA_LIST
+#if !defined(_VA_LIST) && !defined(_VA_LIST_T)
 typedef __builtin_va_list va_list;
 #define _VA_LIST
 #endif

 #ifndef __GNUC_VA_LIST
 #define __GNUC_VA_LIST 1
 typedef __builtin_va_list __gnuc_va_list;
+#define _VA_LIST_DEFINED
 #endif
 
 #endif /* __STDARG_H */

lib/Headers/stddef.h

 #if !__has_feature(modules)
 #define _PTRDIFF_T
 #endif
+
+#if !defined(_PTRDIFF_T_DEFINED)
 typedef __PTRDIFF_TYPE__ ptrdiff_t;
+#define _PTRDIFF_T_DEFINED
+#endif
+
 #endif
 #undef __need_ptrdiff_t
 #endif /* defined(__need_ptrdiff_t) */

 #if !__has_feature(modules)
 #define _SIZE_T
 #endif
+
+#if !defined(_SIZE_T_DEFINED_)
+#if !defined(_SIZE_T_DEFINED)
+#if !defined(_BSD_SIZE_T_DEFINED_)
+#if !defined(_SIZE_T_DECLARED)
 typedef __SIZE_TYPE__ size_t;
+#define _SIZE_T_DEFINED_
+#define _SIZE_T_DEFINED
+#define _BSD_SIZE_T_DEFINED_
+#define _SIZE_T_DECLARED
+#endif
+#endif
+#endif
+#endif
+
+
+
 #endif
 #undef __need_size_t
 #endif /*defined(__need_size_t) */

 #define _WCHAR_T_DEFINED
 #endif
 #endif
+
+#if !defined(__INT_WCHAR_T_H)
+#if !defined(_GCC_WCHAR_T)
+#if !defined(_WCHAR_T_DECLARED)
+#if !defined(_WCHAR_T_DEFINED)
+#define __INT_WCHAR_T_H
+#define _GCC_WCHAR_T
+#define _WCHAR_T_DECLARED
+#define _WCHAR_T_DEFINED
 typedef __WCHAR_TYPE__ wchar_t;
+#endif
+#endif
+#endif
+#endif
+
 #endif
 #endif
 #undef __need_wchar_t