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From: "iains at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/96251] co_yield incorrectly rejected in
 non-explicitly-constexpr generic lambda
Date: Tue, 16 Feb 2021 08:35:54 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D96251

--- Comment #4 from Iain Sandoe <iains at gcc dot gnu.org> ---
So, as noted, the problem is being caused because the coroutine is being
regarded as potentially constexpr while still type-dependent, and then fail=
ing
during template expansion.

All the coroutine expressions are correctly marked as not suitable for
constexpr.

One can also use the DECL_COROUTINE_P on the function decl (which is added =
as
soon as any coroutine keyword is encountered - that would perhaps short-cir=
cuit
some work), but it doesn't fix the bug. ... like so.

diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index 377fe322ee8..c4226599072 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -7827,6 +7827,9 @@ potential_constant_expression_1 (tree t, bool want_rv=
al,
bool strict, bool now,
   switch (TREE_CODE (t))
     {
     case FUNCTION_DECL:
+      if (DECL_COROUTINE_P (t))
+       return false;
+       /* FALLTHROUGH.  */
     case BASELINK:
     case TEMPLATE_DECL:
     case OVERLOAD:

=3D=3D=3D

The for loop body is never visited during the initial parse, but rather the
dependent type in the loop init expression causes a conservative early exit
(with a 'true' result).

It seems that this conservative approach to potentially-constexpr, means th=
at
some way of punting in template expansion is needed (or the initial check n=
eeds
to be less conservative).=