[Bug c++/124275] New: GCC rejects trivially copying a non-constexpr class object containing nullptr_t in constant initialization

[de34 at live dot cn]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=124275

            Bug ID: 124275
           Summary: GCC rejects trivially copying a non-constexpr class
                    object containing nullptr_t in constant initialization
           Product: gcc
           Version: 16.0
            Status: UNCONFIRMED
          Keywords: rejects-valid
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: de34 at live dot cn
  Target Milestone: ---

GCC rejects this following code snippet (https://godbolt.org/z/qYdxjPjWo):
```
struct X {
    constexpr X() = default;
    constexpr X(const X&) = default;

    decltype(nullptr) n;
};

extern X x;
constexpr X y = x;
```

But it accepts this one with the copy constructor is made non-trivial (while
still doing the same thing) (https://godbolt.org/z/z3edEWrYM):
```
struct X {
    constexpr X() = default;
    constexpr X(const X& other) noexcept : n(other.n) {}

    decltype(nullptr) n;
};

extern X x;
constexpr X y = x;
```

Per https://eel.is/c++draft/expr.const ,
> - an lvalue-to-rvalue conversion unless it is applied to
>   - a glvalue of type cv std​::​nullptr_t,

it should be valid to copy a non-constant `nullptr_t` object in constant
evaluation. Given the initialization of `y` selects a `constexpr` constructor,
it should be valid.

This might be related to PR 118661 but is not the same. It's strange that MSVC
and Clang behave similarly. Corresponding bugs are already reported.