[llvm-bugs] [Bug 40559] New: Predefined __FLT16_MANT_DIG__ defined on targets that don't support _Float16

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https://bugs.llvm.org/show_bug.cgi?id=40559

            Bug ID: 40559
           Summary: Predefined __FLT16_MANT_DIG__ defined on targets that
                    don't support _Float16
           Product: clang
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Frontend
          Assignee: unassignedclangbugs at nondot.org
          Reporter: nemanja.i.ibm at gmail.com
                CC: llvm-bugs at lists.llvm.org, neeilans at live.com,
                    richard-llvm at metafoo.co.uk

The Target Info has a query for whether the target supports the _Float16 type.
However, we are not considering that query and predefining __FLT16_MANT_DIG__
unconditionally since rC313152.
This can get us in trouble for some type_traits-like compilations.

Specifically, on PPC:

$ cat predef.c 
#ifdef __FLT16_MANT_DIG__
_Float16 t;
#endif
int main(void) { return 5; }

$ clang predef.c -S
predef.c:2:1: error: _Float16 is not supported on this target
_Float16 t;
^
1 error generated.

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