Why return value optimization does not work when return ()

Question

My code is as follows. Why () makes RVO fail?

A fn() {
    A a{};
    return (a); // move constructor of class A works
    return a;   // RVO works
}

int main() {
    A a = fn();
    return 0;
}

Answer 1

This is NRVO, not RVO.

Here is the rule which allows NRVO (class.copy/31):

in a return statement in a function with a class return type, when the expression is the name of a non-volatile automatic object (other than a function or catch-clause parameter) with the same cv- unqualified type as the function return type, the copy/move operation can be omitted by constructing the automatic object directly into the function’s return value

As you can see, in the case of (a), the expression is not a name (because of the added parenthesis), so NRVO is not allowed.