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From: "pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/106211] Rejects valid with function template with
 non-deduced parameters from deduced parameters of another function template
Date: Wed, 06 Jul 2022 22:34:24 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D106211

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
The question comes about variadic templates and deduction of auto.

>Further, all three compilers also accept this minor modification:
This one does need any deduction though so it is not even related.

So let's take the slightly reduced testcase:

template<typename, typename>
concept any =3D true;

auto t =3D []<typename... Ts>(Ts...) {
    return [](any<Ts> auto... b, auto c) {return c;};
}(1);
auto t1 =3D t(2, 3);

We have now:
Ts being <int>
so we get:

                return [](any<int> auto, auto) {};

Which is invalid as any<int> is invalid.
Maybe I misunderstand how this works though.

If we remove the any<Ts> then we get the same failure for clang as GCC:
<source>:7:11: error: no matching function for call to object of type '(lam=
bda
at <source>:5:12)'
auto t1 =3D t(2, 3);
          ^
<source>:5:12: note: candidate function [with b:auto =3D <>, c:auto =3D int=
] not
viable: requires single argument 'c', but 2 arguments were provided
    return [](auto... b, auto c) {return c;};
           ^

Which is to say the problem is the way variadic templates with auto is being
handled. I suspect the concept is being handled incorrectly for clang.=