c++ - 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

From [3.9/5] I found that:

Incompletely-defined object types and?cv void?are?incomplete types

Anyway, for sizeof does not evaluate it's operands, I would have said that sizeof(void()) was a legal expression (actually GCC compiles it and the result is 1).
On the other side, from here, void is not mentioned while discussing sizeof, neither when the types having size 1 are mentioned, nor in the list of the ones having an implementation defined size.

The question is thus: is sizeof(void()) a legal expression?
Is it guaranteed to have size equal to 1?
Or is it a legal expression resulting in an UB and that's all?

Answer 1

void() is a function type (it's a function which takes no arguments and returns nothing), so it's not a valid type in sizeof().