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From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/103991] [12 Regression] Bogus -Wreturn-type with constexpr
 if and local var with destructor
Date: Wed, 12 Jan 2022 16:33:25 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103991

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 wha=
t 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 o=
ut
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 per=
haps
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 woul=
d be
to revert to previous behavior for IF_STMT_CONSTEXPR_P and IF_STMT_CONSTEVA=
L_P,
  if (IF_STMT_CONSTEVAL_P (stmt))
    stmt =3D else_;
  else if (IF_STMT_CONSTEXPR_P (stmt))
    stmt =3D integer_nonzerop (cond) ? then_ ? else_;
  else
    stmt =3D build3 (COND_EXPR, void_type_node, cond, then_, else_);

Jason, thoughts on this?=