struct A{
    enum class C{
        a = 0
    };
};
int main(){
   A a;
   auto c = a.C::a;  //#1
}

> Each enum-name and each unscoped enumerator is declared in the scope that immediately contains the enum-specifier. 

> An enumerator declared in class scope can be referred to using the class member access operators (​::​, . (dot) and -> (arrow)), see [expr.ref].

According to the above rule, A::C::a shouldn't appear in the id-expression of a member access expression, which means `#1` should be ill-formed.