https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90881
--- Comment #5 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
(In reply to Federico Kircheis from comment #4)
> (In reply to Jonathan Wakely from comment #1)
> > Confirmed. We shouldn't give that warning in unevaluated contexts (decltype,
> > sizeof, etc.) because unevaluated operands have no effects at all, with or
> > without the comma operator.
>
> The compiler flag is about unused variables, not statements without effects.
>
> I guess it would be nice for
>
> ----
> int main(){
> decltype(short(), int()) a = 1;
> return a;
> }
> ----
>
> to have a warning that the left operand is unused (as of today), since here
> SFINAE does not kick in:
>
> --
> <source>: In function 'int main()':
> <source>:2:21: warning: left operand of comma operator has no effect
> [-Wunused-value]
> 2 | decltype(short(), int()) a = 1;
> | ^
> --
>
> But maybe there are not enough use-cases to be worth to diagnostic it.
With my patch, we wouldn't warn on this second testcase. But clang++ doesn't
warn, either.
