[Bug c++/96908] New: [c++20] substitution failure if lambda in default template param

[bastien.penavayre at epitech dot eu]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96908

            Bug ID: 96908
           Summary: [c++20] substitution failure if lambda in default
                    template param
           Product: gcc
           Version: 9.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: bastien.penavayre at epitech dot eu
  Target Milestone: ---

code:
```
template<const int * = ([]{}, nullptr)> // 'auto = []{}' would do the same
void func() {}

template<auto = []{}>
void func2() {}

template<class>
void f()
{
    func(); //substituion failure
    func2(); //substituion failure
}

int main()
{
    func(); // ok
    func2(); // ok
}
```

output:
```
source>: In function 'void f()':
<source>:10:10: error: no matching function for call to 'func()'

   10 |     func(); //substituion failure
      |          ^
<source>:2:6: note: candidate: 'template<const int* <anonymous> > void func()'
    2 | void func() {}
      |      ^~~~
<source>:2:6: note:   template argument deduction/substitution failed:
<source>:11:11: error: no matching function for call to 'func2()'
   11 |     func2(); //substituion failure
      |           ^
<source>:5:6: note: candidate: 'template<auto <anonymous> > void func2()'
    5 | void func2() {}
      |      ^~~~~
<source>:5:6: note:   template argument deduction/substitution failed:
Compiler returned: 1
```

can be tested in godbolt: https://godbolt.org/z/xjTPoj