[Bug c++/114163] New: Calling member function of an incomplete type compiles in gcc and does not compile in clang and msvc

[jlame646 at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114163

            Bug ID: 114163
           Summary: Calling member function of an incomplete type compiles
                    in gcc and does not compile in clang and msvc
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jlame646 at gmail dot com
  Target Milestone: ---

The following invalid program(since lambda's body in not complete class
context) is compiled by gcc:

```
struct TestClass {
  void testFinction() {}
  static TestClass* ptr_;
  constexpr static auto funcPtr = +[](){ ptr_->testFinction(); };
};

TestClass* TestClass::ptr_ = new TestClass();

int main() {
//   TestClass::funcPtr();
//   delete TestClass::ptr_;
  return 0;
}
```
Demo https://godbolt.org/z/Yhc8MeWW6

First discovered at
https://stackoverflow.com/questions/78080083/calling-member-function-of-an-incomplete-type-compiles-in-gcc-and-does-not-compi