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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++/99214] New: Incorrect template arguments shown in
 constraint satisfaction failure diagnostic
Date: Tue, 23 Feb 2021 04:16:19 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99214

            Bug ID: 99214
           Summary: Incorrect template arguments shown in constraint
                    satisfaction failure diagnostic
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ppalka at gcc dot gnu.org
  Target Milestone: ---

$ cat testcase.C
template <class T>
struct A {
  template <class U> static void f() requires ([] { return U::fail; }());
  template <class U> static void f();
};

int main() {
  A<int>::f<char>();
}

$ cat testcase.C | g++-11 -std=3Dc++20 -x c++ -
<stdin>: In substitution of =E2=80=98template<class U> static void A<int>::=
f<U>()
requires (<lambda>)() [with U =3D int]=E2=80=99:
<stdin>:8:19:   required from here
<stdin>:3:63: error: =E2=80=98fail=E2=80=99 is not a member of =E2=80=98cha=
r=E2=80=99

In the above diagnostic, the "in substitution of" context line incorrectly
shows "U =3D int" instead of "U =3D char".

GCC 10 shows the correct template arguments:

$ cat testcase.C | g++-10 -std=3Dc++20 -x c++ -
<stdin>: In instantiation of =E2=80=98static void A<T>::f() requires (<lamb=
da>)() [with
U =3D char; T =3D int]=E2=80=99:
<stdin>:8:19:   required from here
<stdin>:3:63: error: =E2=80=98fail=E2=80=99 is not a member of =E2=80=98cha=
r=E2=80=99=