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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/108393] circular concept false-positive
Date: Fri, 13 Jan 2023 16:15:10 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D108393

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Specifically, ADL for __t =3D=3D __u finds this function:

  template<C _Iter>
  friend constexpr bool operator=3D=3D(unreachable_sentinel_t, const _Iter&)
noexcept;

making it an overload resolution candidate. It's not viable, because __t and
__u (which are the same type) cannot be converted to unreachable_sentinel_t.
But if the other parameter is checked first, then that means checking wheth=
er
S<unreachable_sentinel_t> satisfies C, because of the constraint C _Iter.

[over.match.viable] says:

- First, to be a viable function, a candidate function shall have enough
parameters to agree in number with the arguments in the list.

This condition is met.

- Second, for a function to be viable, if it has associated constraints
([temp.constr.decl]), those constraints shall be satisfied

The associated constraints are that _Iter satisfies C. This is where the cy=
cle
happens.

- Third, for F to be a viable function, there shall exist for each argument=
 an
implicit conversion sequence that converts that argument to the correspondi=
ng
parameter of F.

This is when the function would be discarded from the candidate set, because
S<unreachable_sentinel_t> cannot be converted to unreachable_sentinel_t. But
it's too late, we already blew up at the second step.=