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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++/95010] New: Recursive function template with function
 parameter of type decltype([]{})  is rejected
Date: Fri, 08 May 2020 14:39:23 +0000
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X-Bugzilla-Component: c++
X-Bugzilla-Version: 10.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D95010

            Bug ID: 95010
           Summary: Recursive function template with function parameter of
                    type decltype([]{})  is rejected
           Product: gcc
           Version: 10.0
            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<typename T>
void foo(decltype([]{}) *s) {
  if (0)
    foo<T>(s);
}

void bar() {
  foo<int>(nullptr);
}

$ g++ -std=3Dc++2a testcase.C
testcase.C: In instantiation of =E2=80=98void foo(decltype (<lambda>)*) [wi=
th T =3D int;
decltype (<lambda>) =3D foo<int>::<lambda()>]=E2=80=99:
testcase.C:8:19:   required from here
testcase.C:4:12: error: cannot convert =E2=80=98<lambda()>*=E2=80=99 to =E2=
=80=98<lambda()>*=E2=80=99
    4 |     foo<T>(s);
      |            ^
      |            |
      |            <lambda()>*
testcase.C:2:26: note:   initializing argument 1 of =E2=80=98void foo(declt=
ype
(<lambda>)*) [with T =3D int; decltype (<lambda>) =3D foo<int>::<lambda()>]=
=E2=80=99
    2 | void foo(decltype([]{}) *s) {
      |          ~~~~~~~~~~~~~~~~^


We should also accept:

template<typename T>
void foo(decltype([]{}) *s, decltype(s) q) {
  foo<T>(q, s);
}

void bar() {
  foo<int>(nullptr, nullptr);
}=