[Bug c++/99864] New: Abbreviated member function template doesn't compile with default function argument

["stream009 at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99864

            Bug ID: 99864
           Summary: Abbreviated member function template doesn't compile
                    with default function argument
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: stream009 at gmail dot com
  Target Milestone: ---

I got a strange compile error when I use abbreviated function template and
default function argument on member function.

Compiler version: gcc 10.2
Compiler Exploer Link: https://godbolt.org/z/7cMTP875E

// begin source
class foo
{
public:
    int mem_fn(auto a, int b = 0);
};

inline int foo::
mem_fn(auto a, int b/*= 0*/)
{
    return a + b;
}

int main()
{
    foo x;
    x.mem_fn(1);
}
// end source

// begin error message
16:15: error: call to 'int foo::mem_fn(auto:2, int) [with auto:1 = int]' uses
the default argument for parameter 2, which is not yet defined
   16 |     x.mem_fn(1);
      |               ^
// end error message

- It does compile if function definition is in the class definition
    (https://godbolt.org/z/T3ncebTqz)
- It does compile if function is not member function
    (https://godbolt.org/z/o8Wc98xE4)
- It does compile if member function is not function template
    (https://godbolt.org/z/vWTxTbjoY)
- It does compile if member function is not abbreviated template function
    (https://godbolt.org/z/qqzK9E6qc)