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From: "rs2740 at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/100374] New: Type-constraints of member function templates
 should not be substituted into during implicit instantiation
Date: Sat, 01 May 2021 13:48:12 +0000
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X-Bugzilla-Version: 12.0
X-Bugzilla-Keywords: rejects-valid
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D100374

            Bug ID: 100374
           Summary: Type-constraints of member function templates should
                    not be substituted into during implicit instantiation
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Keywords: rejects-valid
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rs2740 at gmail dot com
  Target Milestone: ---

template<class T, class U>
concept C =3D true;

template <class T>
struct Foo
{
  template <C<typename T::value_type> U>
  void bar()
  {
  }
};

Foo<float> f;

GCC trunk with -std=3Dc++20 produces:

<source>: In instantiation of 'struct Foo<float>':
<source>:13:12:   required from here
<source>:8:8: error: 'float' is not a class, struct, or union type
    8 |   void bar()
      |        ^~~

[temp.inst]/17 says that type-constraints like C<typename T::value_type> sh=
ould
not be substituted into during the implicit instantiation of the declaratio=
n of
Foo<float>::bar:

The type-constraints and requires-clause of a template specialization or me=
mber
function are not instantiated along with the specialization or function its=
elf,
even for a member function of a local class; substitution into the atomic
constraints formed from them is instead performed as specified in
[temp.constr.decl] and [temp.constr.atomic] when determining whether the
constraints are satisfied or as specified in [temp.constr.decl] when compar=
ing
declarations.=