Is sizeof(void()) a legal expression?

Question

From [5.3.3/1], I found that:

The sizeof operator shall not be applied to an expression that has function or incomplete type

Fro

Answer 1

From looking at CppReference.com - sizeof operator, the documentation literally states:

sizeof cannot be used with function types, incomplete types, or bit-field glvalues.

And since void() is a function type, then sizeof(void()) is not a legal expression.

In their usage example, we can see their error comment on this line:

std::cout << "size of function: " << sizeof(void()) << '\n'; // error