C++ decltype and parentheses – why?

It’s not an oversight. It’s interesting, that in Decltype and auto (revision 4) (N1705=04-0145) there is a statement:

The decltype rules now explicitly state that decltype((e)) == decltype(e)(as suggested by EWG).

But in Decltype (revision 6): proposed wording (N2115=06-018) one of the changes is

Parenthesized-expression inside decltype is not considered to be an id-expression.

There is no rationale in the wording, but I suppose this is kind of extension of decltype using a bit different syntax, in other words, it was intended to differentiate these cases.

The usage for that is shown in C++draft9.2.8.4:

const int&& foo();
int i;
struct A { double x; };
const A* a = new A();
decltype(foo()) x1 = 17;        // type is const int&&
decltype(i) x2;                 // type is int
decltype(a->x) x3;              // type is double
decltype((a->x)) x4 = x3;       // type is const double&

What is really interesting, is how it works with the return statement:

decltype(auto) f()
{
    int i{ 0 };
    return (i);
}

My Visual Studio 2019 suggest me to remove redundant parenthesis, but actually they turn into decltype((i)) which changes return value to int& which makes it UB since returning reference to a local variable.