[Bug c++/86494] New: Usage in unevaluated context causes compile time errors because of implicit deletion

["antoshkka at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86494

            Bug ID: 86494
           Summary: Usage in unevaluated context causes compile time
                    errors because of implicit deletion
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: antoshkka at gmail dot com
  Target Milestone: ---

Following code:


template<typename T1>
struct pair {
    T1 first;

    pair() = default;
    pair(const pair&) = default;
    pair(pair&&) = default;

    pair& operator=(pair&& __p);
    pair& operator=(const pair& __p);
};

struct Idx : public pair<int> {
    using pair::pair;
    using pair::operator=;
};

template<typename _Tp> _Tp&& declval() noexcept;

int main() {
    // Comment out the next line and everything will compile
    using t = decltype(declval<Idx>() = declval<Idx>());

    Idx p{};
    Idx p2(p);
}


Fails to compile on GCC with the following error:

<source>: In function 'int main()':
<source>:25:13: error: use of deleted function 'constexpr Idx::Idx(const Idx&)'
     Idx p2(p);
             ^
<source>:13:8: note: 'constexpr Idx::Idx(const Idx&)' is implicitly declared as
deleted because 'Idx' declares a move constructor or move assignment operator
 struct Idx : public pair<int> {
        ^~~

However clang compiles that code well.

With line 'using t =...' commented out everything compiles well on GCC.