[Bug libstdc++/67932] Incorrect conversion to hexfloat

["redi at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67932

--- Comment #7 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The difference will probably be that when including libstdc++'s <cstdio> this
happens before mingw's <stdio.h> is included:

// Make sure that POSIX printf/scanf functions are activated.  As
// libstdc++ depends on POSIX-definitions of those functions, we define
// it unconditionally.
#undef __USE_MINGW_ANSI_STDIO
#define __USE_MINGW_ANSI_STDIO 1

That alters what <stdio.h> declares, due to this in <stdio.h>

#if __USE_MINGW_ANSI_STDIO
/*
 * User has expressed a preference for C99 conformance...
 */

That causes printf to be a wrapper around __mingw_vprintf, rather than calling
the MSVCRT version:

__mingw_ovr
__attribute__((__format__ (gnu_printf, 1, 2))) __MINGW_ATTRIB_NONNULL(1)
int printf (const char *__format, ...)
{
  register int __retval;
  __builtin_va_list __local_argv; __builtin_va_start( __local_argv, __format );
  __retval = __mingw_vprintf( __format, __local_argv );
  __builtin_va_end( __local_argv );
  return __retval;
}

So the incorrect output seems to come from __mingw_vprintf.