[Bug c++/86105] New: Conversion to ambiguous/inaccessible rvalue base is valid in unevaluated context

[johelegp at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86105

            Bug ID: 86105
           Summary: Conversion to ambiguous/inaccessible rvalue base is
                    valid in unevaluated context
           Product: gcc
           Version: 8.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: johelegp at gmail dot com
  Target Milestone: ---

In the following snippet, the assertions fail although the `static_cast` is
ill-formed in non-unevaluated context. See https://godbolt.org/g/45VJvp, and a
related Clang report https://bugs.llvm.org/show_bug.cgi?id=37691.

```C++
#include <utility>

template <class To, class From>
constexpr bool is_static_castable(...)
{
    return false;
};

template <
    class To, class From,
    class = decltype(static_cast<To>(std::declval<From>()))>
constexpr bool is_static_castable(int)
{
    return true;
};

struct B {
};

struct D : B {
};

struct D2
  : D
  , B {
};

static_assert(!is_static_castable<B&&, D2&>(0), "");

struct D3 : private B {
};

static_assert(!is_static_castable<B&&, D3&>(0), "");

void f()
{
    D2 d2;
    (void)static_cast<B&&>(d2);
    D3 d3;
    (void)static_cast<B&&>(d3);
}
```