[Bug c++/63540] New: Erroneous "'Derived' declares a move constructor or move assignment operator" in error.

[brooks at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63540

            Bug ID: 63540
           Summary: Erroneous "'Derived' declares a move constructor or
                    move assignment operator" in error.
           Product: gcc
           Version: 4.9.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: brooks at gcc dot gnu.org

Consider the following reduced testcase:

----
template <typename T, typename = decltype(*static_cast<T*>(0) = 0)> int
break_it();
template <typename> int break_it();

struct Base {
  Base(const Base &);
  void operator=(Base &&);
};
struct Derived : Base {
  using Base::operator=;
};

int a = break_it<Derived>();
Derived v(v);
----

With a recent 4.9, it fails with an erroneous error:

----
$ gcc-archive/4.9/215880/bin/g++ --std=gnu++11 t.cc -c -o t.o
t.cc:13:12: error: use of deleted function ‘Derived::Derived(const Derived&)’
 Derived v(v);
            ^
t.cc:8:8: note: ‘Derived::Derived(const Derived&)’ is implicitly declared as
deleted because ‘Derived’ declares a move constructor or move assignment
operator
 struct Derived : Base {
        ^
----

The same erroneous error occurs on trunk.

The call to 'Derived::operator=' in a SFINAE context within break_it seems to
be confusing GCC about which special members Derived declares.