[Bug c++/95010] New: Recursive function template with function parameter of type decltype([]{}) is rejected

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95010

            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=c++2a testcase.C
testcase.C: In instantiation of ‘void foo(decltype (<lambda>)*) [with T = int;
decltype (<lambda>) = foo<int>::<lambda()>]’:
testcase.C:8:19:   required from here
testcase.C:4:12: error: cannot convert ‘<lambda()>*’ to ‘<lambda()>*’
    4 |     foo<T>(s);
      |            ^
      |            |
      |            <lambda()>*
testcase.C:2:26: note:   initializing argument 1 of ‘void foo(decltype
(<lambda>)*) [with T = int; decltype (<lambda>) = foo<int>::<lambda()>]’
    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);
}