[Bug c++/115656] [11/12/13/14/15 Regression] Templated ctor use rejected in non-deduced context if class template has template template parameter

["s.murthy at outlook dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115656

--- Comment #6 from Sean Murthy <s.murthy at outlook dot com> ---
Additional info, just in case it helps. Either of the following changes to the
repro causes the reported error to disappear (not saying that either change is
universally acceptable, that is, they are not workarounds).

1. In class A, unconstrain the type parameter of template template parameter V,
and make no other changes anywhere else.

```
template<std::unsigned_integral U = unsigned,
         template <class> class V = dv
        >
```

2. In class A, constrain the type parameter of template template parameter V
using type_traits *and* unconstrain the type parameter of class template dv:

```
template<typename> class dv;

template<std::unsigned_integral U = unsigned,
         template <typename T> requires std::is_unsigned_v<T> class V = dv
        >
```

Furthermore, constraining the type parameter of class template dv using
type_traits instead of concepts causes *CTAD fail* for class A and non-CTAD
fail for class B (which is odd because the change has nothing to do with class
B). And these failures go back to GCC 10.1. Assuming purely C++20 issue, this
particular manifestation does not appear to be regression.

See: https://sigcpp.godbolt.org/z/79oEaG1vo