[Bug c++/96848] New: Inherited conditionally explicit constructors via using declaration do not enforce explicitness if dependent on template parameter

["northon_patrick3 at yahoo dot ca" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96848

            Bug ID: 96848
           Summary: Inherited conditionally explicit constructors via
                    using declaration do not enforce explicitness if
                    dependent on template parameter
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: northon_patrick3 at yahoo dot ca
  Target Milestone: ---

Example: https://godbolt.org/z/4x9r7e

```cpp
#include <iostream>
#include <type_traits>

struct E {};
struct F {};
struct G {};

template <typename From_, typename To_>
constexpr bool isValidImplicitConvert = false;
template <>
constexpr bool isValidImplicitConvert<E, G> = true;

template <typename T_>
struct A
{
        template <typename OT_>
        explicit(!isValidImplicitConvert<OT_, T_>) constexpr A(const OT_ &) {}

        //explicit(isValidImplicitConvert<E, T_>) constexpr A(E) {}
        //explicit(isValidImplicitConvert<F, T_>) constexpr A(F) {}
};

template <typename T_>
struct B : public A<T_>
{
        using A<T_>::A;
};

int main(int, char **)
{
        std::cout << std::is_convertible_v<E, A<G>> << '\n';
        std::cout << std::is_convertible_v<F, A<G>> << '\n';

        std::cout << std::is_convertible_v<E, B<G>> << '\n';
        std::cout << std::is_convertible_v<F, B<G>> << '\n';

        return 0;
}
```

Output:
```
1
0
1
1
```

Expected output:
```
1
0
1
0
```

When inheriting constructors from a base class via a using declaration, if said
constructors are conditionally explicit and the condition is dependent on a
deduced template parameter, the explicit state is ignored.

Clang give the expected output, however I am not sure who is correct.