[Bug c++/103991] [12 Regression] Bogus -Wreturn-type with constexpr if and local var with destructor

["rguenther at suse dot de" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103991

--- Comment #7 from rguenther at suse dot de <rguenther at suse dot de> ---
> Am 12.01.2022 um 17:33 schrieb jakub at gcc dot gnu.org <gcc-bugzilla@gcc.gnu.org>:
> 
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103991
> 
> Jakub Jelinek <jakub at gcc dot gnu.org> changed:
> 
>           What    |Removed                     |Added
> ----------------------------------------------------------------------------
>                 CC|                            |rguenth at gcc dot gnu.org
> 
> --- Comment #4 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
> For IF_STMT_CONSTEXPR_P and IF_STMT_CONSTEVAL_P IF_STMTs, it is unclear what we
> should do, because in either case we throw away the other branch if any.
> Either we do for those what we used to do before r12-5638 and risk
> -Wunreachable-code warnings (when/if it is readded), e.g. on code like:
>  if constexpr (true)
>    return 0;
>  some code;
> but we don't emit these -Wreturn-type false positives in cases where the
> untaken block of code doesn't fall through.
> Or the r12-5638 can result in such false positives.
> Or perhaps we should track if we had the other block of code at all (if not, it
> is ok to do what we do right now) and if possible otherwise try to figure out
> if the other block could fall through and if it can't, perhaps replace the
> void_node with __builtin_unreachable () call?
> For IF_STMT_CONSTEVAL_P we still have the other branch around and could perhaps
> call block_may_fallthru on it, but for IF_STMT_CONSTEXPR_P we discard it
> earlier,
> outside of templates already during parsing.
> 
> Now, as Richi's warning isn't in GCC 12, quickest/safest temporary fix would be
> to revert to previous behavior for IF_STMT_CONSTEXPR_P and IF_STMT_CONSTEVAL_P,
>  if (IF_STMT_CONSTEVAL_P (stmt))
>    stmt = else_;
>  else if (IF_STMT_CONSTEXPR_P (stmt))
>    stmt = integer_nonzerop (cond) ? then_ ? else_;
>  else
>    stmt = build3 (COND_EXPR, void_type_node, cond, then_, else_);

I agree that reverting for GCC 12 is the most reasonable thing with adding a
Testcase

> Jason, thoughts on this?
> 
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