Are nested parentheses allowed in function type alias declaration?

Question

The following code can be compiled with gcc-13 and clang-16, but MSVC reports multiple errors

using foo_type = void ((((int, const char*))));   // (1)
// Compiler messages for line (1):
//    clang-16: ok.
//    gcc-13:   ok.
//    MSVC-v19: error C2062: type 'int' unexpected.


foo_type foo;

int main()
{
    foo(5, "Hello");
    return 0;
}

The question is: who's right? Does the Standard allow alias-declaration of a function type with multiple nested parentheses around argument list, like attempted in the example above?

Answer 1

Yes, that syntax is allowed and not specific to alias declarations. It applies to all declarators.

You can generally put as many parentheses around a declarator as you want. In the case of an abstract-declarator such as here, nothing changes, except that the declarator-id (i.e. the name) is omitted. An exception are declarators with a trailing-return-type.

For example, the following is also valid:

void (((foo(int, const char*))));

int main()
{
    foo(5, "Hello");
    return 0;
}

For the same reason you can also add parentheses around the * abstract-declarator in the function parameter:

using foo_type = void ((((int, const char(((*)))))));

Specifically for the language-lawyer tag, see the grammar production ( ptr-declarator ) for noptr-declarator in the non-abstract case and ( ptr-abstract-declarator ) for noptr-abstract-declarator in the abstract case.