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From: "pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/95968] error: 'args#0' is not a constant expression
Date: Thu, 19 Jan 2023 01:26:41 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D95968

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|rejects-valid               |

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #2)
> The following is the reduced testcase:
> struct test_class
> {
>     consteval test_class( double pack_) { }
> };
>=20
> void test_func()=20
> {
>     auto t =3D [](const auto... args) {return test_class(args...);};
>     t(1.0);
> }
>=20
> are lambdas consteval by default? I know they are implicit constexpr but I
> don't think they are consteval though.

They are not until C++23 (See PR 107687 for the status on that).

This is just a diagnostic issue. add constevalue like this to the reduced
testcase allows the testcase to work:
    auto t =3D [](const auto... args) consteval {return test_class(args...)=
;};

Note the original testcase still has issues even if you add consteval to the
lambda and I suspect because std::apply still has issues ...=