[Bug c++/93557] New: __builtin_convertvector doesn't mak input as used

[evan@coeus-group.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93557

            Bug ID: 93557
           Summary: __builtin_convertvector doesn't mak input as used
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: evan@coeus-group.com
  Target Milestone: ---

Created attachment 47771
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=47771&action=edit
test case (pass -Wextra to enable the warnings)

I'm getting some false positives with -Wunused-but-set-parameter and
-Wunused-but-variable which I've narrowed down to __builtin_convertvector. 
There is a quick test case attached, or ot godbolt at
https://godbolt.org/z/RSFLGB

It works as expected with gcc, but with g++ I get:

    cv.cc: In function ‘vecf conv(veci)’:
    cv.cc:4:16: warning: parameter ‘a’ set but not used
[-Wunused-but-set-parameter]
        4 | vecf conv(veci a) {
          |           ~~~~~^
    cv.cc: In function ‘vecf conv2(veci)’:
    cv.cc:11:8: warning: variable ‘tmp’ set but not used
[-Wunused-but-set-variable]
       11 |   veci tmp = a;
          |        ^~~

In case anyone else finds this, as a (hopefully temporary) workaround I'm
planning to modify my macro wrapper to use a statement expr to create a
temporary variable which I mark as used using the ((void) foo) trick:

#define SIMDE__CONVERT_VECTOR(to, from) ((to) = (__extension__({ \
     __typeof__(from) from_ = (from); \
     ((void) from_); \
     __builtin_convertvector(from_, __typeof__(to)); \
   })))