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From: "ppalka at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/107461] [12/13 Regression] ambiguity error for friend with
 templated constexpr argument
Date: Sat, 04 Feb 2023 17:02:57 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107461

--- Comment #9 from Patrick Palka <ppalka at gcc dot gnu.org> ---
(In reply to Jakub Jelinek from comment #7)
> Reduced testcase:

Interestingly Clang also rejects this testcase, so I'm not sure if we were
correct to accept it previously.

Here's a more reduced testcase that seems clearly valid:

template <typename T> T foo ();

template <typename> struct A { };

template <typename T> struct B {
  template <typename U, typename =3D A<decltype (foo<T>() (U()))>>
  static void bar(U);
};

int main() {
  B<int> b;
  B<void(*)(int)>::bar(0);
}

<stdin>: In function =E2=80=98int main()=E2=80=99:
<stdin>:12:23: error: no matching function for call to =E2=80=98B<void
(*)(int)>::bar(int)=E2=80=99
<stdin>:7:15: note: candidate: =E2=80=98template<class U, class> static void
B<T>::bar(U) [with <template-parameter-2-2> =3D U; T =3D void (*)(int)]=E2=
=80=99
<stdin>:7:15: note:   template argument deduction/substitution failed:
<stdin>:6:54: error: expression cannot be used as a function

If we remove the line #1 then this bogus error disappears.=