Does a reference have a storage location?

The latest C++20 spec(§ 9.2.3.3) and at least since the C++ 2005 draft spec state:

It is unspecified whether or not a reference requires storage

The actual implementation is on a case-by-case basis. Obviously if a class has a single member variable that is a reference that will need to be stored somewhere. But the compiler has leeway when to use a reference solely as an alias, as you put it.

Most compilers, for any C++ standard up to C++17 at least, will effectively implement a reference as a pointer, unless optimized out.

In particular, inside an struct, it will take take up the size of a pointer (plus alignment/padding etc.).

Therefore, this will hold in most environments:

struct S {
    char & a;
};

static_assert(sizeof(S) == sizeof(void *));