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From: "ppalka at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/95132] Concept checked after auto return type deduction
Date: Sun, 25 Oct 2020 19:27:19 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D95132

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|WAITING                     |ASSIGNED
           Assignee|unassigned at gcc dot gnu.org      |ppalka at gcc dot g=
nu.org
   Target Milestone|---                         |10.3

--- Comment #3 from Patrick Palka <ppalka at gcc dot gnu.org> ---
(In reply to Patrick Palka from comment #2)
> (In reply to Francesco Biscani from comment #0)
> > This is problematic because it means that another concept that would ch=
eck
> > whether or not bar() can be called with a specific argument type would =
fail
> > with a hard compile time error, instead of marking the concept as not
> > satisfied.
>=20
> Would you have a concrete testcase for this behavior?

Ah, here's one:

$ cat testcase.C
template<typename T> struct A {
  static auto f() requires false { return T::fail; }
};

template<typename T>
constexpr int v =3D requires { A<T>::f(); };

static_assert(!v<int>);

$ g++ -std=3Dc++20 testcase.C
testcase.C: In instantiation of =E2=80=98static auto A<T>::f() requires  fa=
lse [with T
=3D int]=E2=80=99:
testcase.C:   required from =E2=80=98constexpr const int v<int>=E2=80=99
testcase.C:   required from here
testcase.C: error: =E2=80=98fail=E2=80=99 is not a member of =E2=80=98int=
=E2=80=99
    2 |   static auto f() requires false { return T::fail; }
      |                                              ^~~~


We wrongly issue a hard error for the reason you pointed out.=