Why forward declaration is allowed inside function argument list?

Question

This code shouldn't compile in my opinion, but it seems compiler treats struct NonExistingNeverDeclaredType* argument as a forward declaration (proof). But why?

#include <iostream>

int foo(struct NonExistingNeverDeclaredType* arg) {
  return sizeof(arg);
}

int main() {
  std::cout << foo(nullptr) << std::endl;
  return 0;
}

https://ideone.com/v3XzCw

Answer 1

This is a property of the elaborated type specifier, it will introduce a declaration if the type is not previously declared:

cppreference

If the name lookup does not find a previously declared type name, the elaborated-type-specifier is introduced by class, struct, or union (i.e. not by enum), and class-name is an unqualified identifier, then the elaborated-type-specifier is a class declaration of the class-name.

It doesn't matter whether it's inside a function argument list or not. The following code is also valid:

class foo {
    class bar *b; // bar is not previously declared
};