[Bug c/93410] New: can't use _Decimal64 in C99/C11/C17 mode

[vincent-gcc at vinc17 dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93410

            Bug ID: 93410
           Summary: can't use _Decimal64 in C99/C11/C17 mode
           Product: gcc
           Version: 9.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vincent-gcc at vinc17 dot net
  Target Milestone: ---

With GCC 9.2.1 under Debian/unstable (x86_64):

cventin% cat tst.c
int main (void)
{
  _Decimal64 x = 1;
  return x != 1;
}
cventin% gcc-9 tst.c -o tst 
cventin%

but

cventin% gcc-9 -std=c99 tst.c -o tst
tst.c: In function ‘main’:
tst.c:3:3: error: unknown type name ‘_Decimal64’
    3 |   _Decimal64 x = 1;
      |   ^~~~~~~~~~
cventin% gcc-9 -std=c11 tst.c -o tst
tst.c: In function ‘main’:
tst.c:3:3: error: unknown type name ‘_Decimal64’
    3 |   _Decimal64 x = 1;
      |   ^~~~~~~~~~
cventin% gcc-9 -std=c17 tst.c -o tst
tst.c: In function ‘main’:
tst.c:3:3: error: unknown type name ‘_Decimal64’
    3 |   _Decimal64 x = 1;
      |   ^~~~~~~~~~
cventin%

There is no such issue with the trunk (where one just gets a warning if
-pedantic is used, which is expected).