[Bug c++/67225] New: [concepts] Expression constraint with a constrained result turns off access checking

[ville.voutilainen at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67225

            Bug ID: 67225
           Summary: [concepts] Expression constraint with a constrained
                    result turns off access checking
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Keywords: accepts-invalid
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ville.voutilainen at gmail dot com
                CC: andrew.n.sutton at gmail dot com, eric.niebler at gmail dot com,
                    jason at redhat dot com
  Target Milestone: ---

Test, which compiles successfully but probably shouldn't:

template <class T, class U> 
concept bool Same() 
{
    return true;
}

template <class T> struct WrapT {T t;};

template <class T>
concept bool Destructible()
{
    return requires(T t, const T ct, WrapT<T>& wt)
    {
        {wt.~WrapT()} noexcept;
        {&t} -> Same<T*>; // #1
        //{&t} -> T*; // #2
    };
}

template <Destructible T>
void f() {}

struct Y {private: ~Y();};

int main()
{
    f<Y>();
}

If #1 is commented out, f<Y>() will correctly fail. Whether #2
is present doesn't matter. If the destructor of Y is deleted,
the presence of #1 doesn't matter and f<Y>() is rejected.