Clang rejects valid program involving member access operator with incomplete type CWG 1836

[ranaanoop]

The following valid program as explained here is rejected by clang and msvc. Demo

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;
}

Clang says:

<source>:4:46: error: member access into incomplete type 'TestClass'
    4 |   constexpr static auto funcPtr = +[](){ ptr_->testFinction(); };
      |                                              ^
<source>:1:8: note: definition of 'TestClass' is not complete until the closing '}'
    1 | struct TestClass {
      |        ^
1 error generated.
Compiler returned: 1

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Anoop Rana (ranaanoop)

The following valid program as explained [here](https://stackoverflow.com/a/78080150/12002570) is rejected by clang and msvc. [Demo](https://godbolt.org/z/Yhc8MeWW6)

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;
}

Clang says:

<source>:4:46: error: member access into incomplete type 'TestClass'
    4 |   constexpr static auto funcPtr = +[](){ ptr_->testFinction(); };
      |                                              ^
<source>:1:8: note: definition of 'TestClass' is not complete until the closing '}'
    1 | struct TestClass {
      |        ^
1 error generated.
Compiler returned: 1