US043 06.08 [basic.def].2.17 "using enum" feature is too subtle #42
Description
06.08 [basic.def] 9 [dcl.dcl] 9.1.8.3 [dcl.type.elab] 9.6.2 [enum.udecl] 9.8 [namespace.udecl] 11.3 [class.mem] 15.10 [cpp.predefined]
The syntax of the using enum feature is very subtle, which may be confusing to users. Consider, for example:
struct B {
protected:
enum class E { e1, e2 };
};
class D1 : public B {
public:
using B::E;
// Creates a public member using
// declaration in 'D1' for 'B::E'.
};
class D2 : public B {
public:
using enum B::E;
// Creates a public member using
// declarations in 'D2' for
// 'E::e1' and 'E::e2'.
};
void f() {
D1::E v1; // Ok
D2::E v2; // COMPILE FAILURE
D1::e1; // COMPILE FAILURE
D2::e2; // Ok
}
Additionally, the syntax looks similar to elaborated type specifiers, which may also lead to confusion. Please reconsider this feature; it was added very late in the C++20 design process, and it may be better to move it to C++23 to give us more time to work on the design.