maintainer-feedback requested: [Bug 274042] lang/gcc8 lang/gcc9: fix build with libc++ 17

Bugzilla Automation <bugzilla@FreeBSD.org> has asked freebsd-toolchain (Nobody)
<toolchain@FreeBSD.org> for maintainer-feedback:
Bug 274042: lang/gcc8 lang/gcc9: fix build with libc++ 17
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=274042



--- Description ---
When building relatively recent gcc ports (with C++ in them) against libc++ 17,
you get errors similar to:

In file included from
/wrkdirs/usr/ports/lang/gcc12/work/gcc-12.2.0/gcc/cp/module.cc:208:
In file included from
/wrkdirs/usr/ports/lang/gcc12/work/gcc-12.2.0/gcc/system.h:239:
In file included from /usr/include/c++/v1/vector:321:
In file included from /usr/include/c++/v1/__format/formatter_bool.h:20:
In file included from /usr/include/c++/v1/__format/formatter_integral.h:32:
In file included from /usr/include/c++/v1/locale:202:
/usr/include/c++/v1/__locale:546:5: error: '__abi_tag__' attribute only applies
to structs, variables, functions, and namespaces
  546 |     _LIBCPP_INLINE_VISIBILITY
      |     ^
/usr/include/c++/v1/__config:813:37: note: expanded from macro
'_LIBCPP_INLINE_VISIBILITY'
  813 | #  define _LIBCPP_INLINE_VISIBILITY _LIBCPP_HIDE_FROM_ABI
      | 				    ^
/usr/include/c++/v1/__config:792:26: note: expanded from macro
'_LIBCPP_HIDE_FROM_ABI'
  792 | 	 
__attribute__((__abi_tag__(_LIBCPP_TOSTRING(_LIBCPP_VERSIONED_IDENTIFIER))))
      | 			 ^
In file included from
/wrkdirs/usr/ports/lang/gcc12/work/gcc-12.2.0/gcc/cp/module.cc:208:
In file included from
/wrkdirs/usr/ports/lang/gcc12/work/gcc-12.2.0/gcc/system.h:239:
In file included from /usr/include/c++/v1/vector:321:
In file included from /usr/include/c++/v1/__format/formatter_bool.h:20:
In file included from /usr/include/c++/v1/__format/formatter_integral.h:32:
In file included from /usr/include/c++/v1/locale:202:
/usr/include/c++/v1/__locale:547:37: error: expected ';' at end of declaration
list
  547 |     char_type toupper(char_type __c) const
      | 				    ^
/usr/include/c++/v1/__locale:553:48: error: too many arguments provided to
function-like macro invocation
  553 |     const char_type* toupper(char_type* __low, const char_type* __high)
const
      | 					       ^
/wrkdirs/usr/ports/lang/gcc12/work/gcc-12.2.0/gcc/../include/safe-ctype.h:146:9
:
note: macro 'toupper' defined here
  146 | #define toupper(c) do_not_use_toupper_with_safe_ctype
      | 	^

This is because gcc/system.h includes safe-ctype.h which redefines ctype macros
such as toupper, tolower, etc to "poison" them.

However, it should only include the safe-ctype.h header *after* any C++
headers, such as <list>, <map>, <string>, etc, otherwise these might
transitively include internal ctype headers (such as with libc++ 17), causing
the above conflicts.

Here is a patch that moves the safe-ctype.h inclusion to later in gcc/system.h,
which solves this issue, and makes it possible to build against libc++ 17.