Clang rejects valid program involving calling of a member function of an incomplete type inside class definition

[ranaanoop]

The following valid program is rejected by clang and msvc but accepted by gcc. Demo

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

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

int main() {
  TestClass::funcPtr();
  delete TestClass::ptr_;
  return 0;
}

Clang says:

<source>:18:46: error: member access into incomplete type 'TestClass'
   18 |   constexpr static auto funcPtr = +[](){ ptr_->testFunction(); };
      |                                              ^
<source>:15:8: note: definition of 'TestClass' is not complete until the closing '}'
   15 | struct TestClass {
      |        ^

expr.post#expr.ref and CWG 1836 makes this well-formed.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Anoop Rana (ranaanoop)

The following valid program is rejected by clang and msvc but accepted by gcc. [Demo](https://godbolt.org/z/qGaxc1Ga5)

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

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

int main() {
  TestClass::funcPtr();
  delete TestClass::ptr_;
  return 0;
}

Clang says:

<source>:18:46: error: member access into incomplete type 'TestClass'
   18 |   constexpr static auto funcPtr = +[](){ ptr_->testFunction(); };
      |                                              ^
<source>:15:8: note: definition of 'TestClass' is not complete until the closing '}'
   15 | struct TestClass {
      |        ^

expr.post#expr.ref and CWG 1836 makes this well-formed.

[keinflue]

This seems to be an exact duplicate of #83424.