Elaborated type specifier in type alias

Why there is no hard error in the following code? Type alias and name of class are exactly the same (compiler clang):

using S = struct S;

struct S {};

S s;

int main()
{
}

Wich name exactly used in the definition of variable in the following code (symbol or type alias)?

using S = struct S {};

int main()
{
    S s;
}