[Bug c/56086] New: when compiling C code with -std=gnu99 macro __STDC_UTF_16__ is defined

["rjarrett at mathworks dot com" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56086

             Bug #: 56086
           Summary: when compiling C code with -std=gnu99 macro
                    __STDC_UTF_16__ is defined
    Classification: Unclassified
           Product: gcc
           Version: 4.7.2
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: rjarrett@mathworks.com


Created attachment 29255
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=29255
sample program

The macro __STDC_UTF_16__ is unexpectedly defined  when compiling  C code using
gcc 4.7.2 (also present in gcc 4.7.1)

This is a regression from gcc 4.5.3. (also gcc 4.4.6)

compilation line:
gcc -c -std=gnu99 gcc47_stdc_utf_16.c 

expected result: no output

actual result:
gcc47_stdc_utf_16.c:3:2: warning: #warning __STDC_UTF_16__ defined [-Wcpp]


use case was in a header file used for both C and C++ compilation that
contained
#if defined(__STDC_UTF_16__)
...
#endif

workaround:
use :#if (__cplusplus && defined(__STDC_UTF_16__)) in place of


#if defined(__STDC_UTF_16__)