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

[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.
>From gcc-bugs-return-464088-listarch-gcc-bugs=gcc.gnu.org@gcc.gnu.org Wed Oct 15 02:42:22 2014
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From: "ppluzhnikov at google dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/63540] Erroneous "'Derived' declares a move constructor or move assignment operator" in error.
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https://gcc.gnu.org/bugzilla/show_bug.cgi?idc540

Paul Pluzhnikov <ppluzhnikov at google dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |ppluzhnikov at google dot com

--- Comment #1 from Paul Pluzhnikov <ppluzhnikov at google dot com> ---
Google ref: b/17785687

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

[redi at gcc dot gnu.org]

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

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |rejects-valid
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2014-10-15
     Ever confirmed|0                           |1

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

[danregister at poczta dot fm]

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

Daniel Adamski <danregister at poczta dot fm> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |danregister at poczta dot fm

--- Comment #2 from Daniel Adamski <danregister at poczta dot fm> ---
I have another variant of that issue:
----------------------------------------------------------------
struct Base
{
    Base() {}
    Base(const Base &) {}
    Base & operator=(Base &&) { return *this; }
};
struct D1 : Base
{
    using Base::operator=;
    D1 & operator=(const D1&) { return *this; }
};

int main()
{
    D1 x;
    D1 y = x;
    (void) y;
}
----------------------------------------------------------------

The error:
----------------------------------------------------------------
test.cpp: In function ‘int main()’:
test.cpp:19:12: error: use of deleted function ‘D1::D1(const D1&)’
     D1 y = x;
            ^
test.cpp:10:8: note: ‘D1::D1(const D1&)’ is implicitly declared as deleted
because ‘D1’ declares a move constructor or move assignment operator
 struct D1 : Base
----------------------------------------------------------------

It's enough to remove one of the lines (any one) from D1 for the error to go
away.

Yet another variant:
----------------------------------------------------------------
struct Base
{
    Base() {}
    Base(const Base &) {}
    Base & operator=(Base &&) { return *this; }
};
struct D1 : Base
{
    using Base::operator=;
};
struct D2 : D1
{
    D2 & operator=(const D2 &) { return *this; }
};

int main()
{
    D1 x;
    D1 y = x;
    (void) y;
}
----------------------------------------------------------------

The error is the same. The "operator=()" in "D2" doesn't have to be D2's
assignment operator. It may some other "operator()", e.g., "operator(int)" or
it may be "using D1::operator=;" or "using Base::operator=;"
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Subject: [Bug c/65901] no warning or error for va_arg (ap, void)
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https://gcc.gnu.org/bugzilla/show_bug.cgi?ide901

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2015-04-27
                 CC|                            |mpolacek at gcc dot gnu.org
   Target Milestone|---                         |6.0
     Ever confirmed|0                           |1

--- Comment #1 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
We should probably reject such a program.  G++/clang reject that as well.