Converting constructor of templated non-type template parameter

Question

When throwing T::v of type int into t<>, I would expect N to be created by use of n's converting constructor. However, for some reason GCC seems to look for member i in c<int, 3>::v. What does the C++20 standard say about this type of construction or deduction for non-type template parameters?

#include <concepts>

template<auto...>
struct n {
    constexpr n(int x): i{x} {}
    int i;
};
template<typename T, T V>
struct c { static constexpr T v = V; };

template<n N>
using t = c<decltype(N.i), N.i>;

// clang ok, gcc nope, msvc ok
static_assert([]<typename T = t<3>>
    { return std::same_as<T, t<T::v>>; }());

Live example

---

The error message from GCC:

<source>: In instantiation of '<lambda()> [with T = c<int, 3>]':
<source>:15:41:   required from here
<source>:15:19: error: request for member 'i' in 'c<int, 3>::v', which is of
non-class type 'const int'
   15 |     { return std::same_as<T, t<T::v>>; }());
      |              ~~~~~^~~~~~~~~~~~~~~~~~~

Answer 1

As per the comments, your understanding is correct and this is definitely a GCC bug. Strictly, the answer to the question is that C++20 says unfortunately little about how the template parameter object is initialized. A defect report has just been approved; until that link works the previous version is available.