The alias-declaration of decltype lambda is broken in the for loop

[hewillk]

Clang accepts the following invalid code.

template<class T>
void f() {
  for (using U = decltype([]{}.operator----); 
       auto x : "abc") { }
};

int main() {
  f<int>();
}

https://godbolt.org/z/dTKbnv3nE

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[shafik]

If we attempt to use the type we crash: https://godbolt.org/z/KdaYW8r3h

In the non-templated case we obtain the correct diagnostic: https://godbolt.org/z/5orxWsfEc

[cor3ntin]

https://reviews.llvm.org/D154492 This ended up a pretty bad bug, we did not preserve the using declaration in a using statement, at all . So when doing a TreeTransform, there was nothing to transform.

The fact this was found by fuzzing is pretty funny to me :D

[cor3ntin]

Fixed!