[Bug c++/128656] New: Accepts method definition of local class outside of the function

[pinskia at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=128656

            Bug ID: 128656
           Summary: Accepts method definition of local class outside of
                    the function
           Product: gcc
           Version: 17.0
            Status: UNCONFIRMED
          Keywords: accepts-invalid
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pinskia at gcc dot gnu.org
  Target Milestone: ---

GCC accepts function definition of a local class from outside of the function:
```
auto f() {
    struct A {
        A();
        void g();
    };
    return A{};
}

using A = decltype(f());

A::A() = default;
void A::g() {}
```

[class.local]/2: "Member functions of a local class shall be defined within
their class definition, if they are defined at all." 

See also https://stackoverflow.com/q/79808341/7325599 and CWG2890.

Note GCC also accepts inner class definition too:
```
void f1() {
  class A { class B; };
  {
    class A::B {};
  } 
}
```