[Bug c++/108588] New: __is_constructible returns wrong value for invalid (but non deleted) default constructor

[Bug c++/108588] New: __is_constructible returns wrong value for invalid (but non deleted) default constructor

[roi.jacobson1 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108588

            Bug ID: 108588
           Summary: __is_constructible returns wrong value for invalid
                    (but non deleted) default constructor
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: roi.jacobson1 at gmail dot com
  Target Milestone: ---

A weird edge case around is_constructible was reported to Clang at
https://github.com/llvm/llvm-project/issues/60293, and we think GCC might have
a bug around invalid and defaulted default constructors:

template <class T>
struct Wrapper {
    T x = (void*)nullptr;
};

static_assert(!__is_constructible(Wrapper<int>));

This code currently compiles under GCC. But by [class.default.ctor#2.7],
Wrapper's default constructor is not deleted and so __is_constructible has no
business returning false - it should either return true or diagnose an error
while instantiating the default constructor that would not be in the immediate
context.

[Bug c++/108588] __is_constructible returns wrong value for invalid (but non deleted) default constructor

[fchelnokov at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108588

--- Comment #1 from Fedor Chelnokov <fchelnokov at gmail dot com> ---
According to this StackOverflow answer, the behavior of GCC is incorrect here:
https://stackoverflow.com/a/75380301/7325599