[Bug c++/90691] [9/10 regression] -Wsign-compare false-positive with constant

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90691

--- Comment #5 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #4)
> (In reply to Jakub Jelinek from comment #3)
> > Started with r270136.  I'd say we should stop considering
> > -fcompare-debug=-W* as bugs, after all, it is extremely unlikely we are
> > going to fix those anyway, warnings do affect code generation in pretty
> > significant way, and just revert that change.
> 
> Err, please no.  -W* should not affect code generation otherwise we can
> simply declare -g may as well for even more obvious reasons.

The choices are live with -fcompare-debug=-W* goal resulting in worse warnings
(like this PR), not considering -fcompare-debug=-W* a bug, slowing compile time
by doing all the maybe_const_value etc. calls regardless if we need them for
warnings or not, or perhaps some middle-ground like ask the constexpr
evaluation to compute something only without trying to instantiate anything
(nor copy_fn anything) when it is called from the warning guarded paths, just
punting in that case.