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

[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]

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.

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

[pinskia at gcc dot gnu.org]

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

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
ICC, clang and MSVC all agree the output should be 1010.

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

[pinskia at gcc dot gnu.org]

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

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2021-08-17
     Ever confirmed|0                           |1
         Depends on|                            |85251

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed. I suspect PR 85251 is the underlying issue though, GCC loses
explicit even in the unconditional case.


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85251
[Bug 85251] Using declaration for base class constructor looses explicit.

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

[oliver.rosten at googlemail dot com]

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

Oliver Rosten <oliver.rosten at googlemail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |oliver.rosten at googlemail dot co
                   |                            |m

--- Comment #3 from Oliver Rosten <oliver.rosten at googlemail dot com> ---
I just came across an issue which I think has the same underlying cause:

https://godbolt.org/z/Ph8xbeM7s