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From: "msebor at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/98835] False positive -Wclass-memaccess with class with
 ref-qualified copy-assignment operator
Date: Tue, 26 Jan 2021 19:38:53 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D98835

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
           Keywords|                            |diagnostic
                 CC|                            |msebor at gcc dot gnu.org
         Resolution|---                         |INVALID

--- Comment #1 from Martin Sebor <msebor at gcc dot gnu.org> ---
The warning is issued on the basis that the left-hand operand of Bad's
assignment cannot be a temporary.  I.e., that this isn't valid: Bad() =3D B=
ad().=20
In the modified test case below the warning detects the memcpy call bypassi=
ng
the constraint the assignment operator puts on assigning to objects of the
class.  So unless there's a compelling use case showing otherwise I'm incli=
ned
to say the warning is justified and helpful here (its wording shouldn't be
expected to perfectly reflect all the subtleties involved in the decision).=
=20
There isn't a test case for this in the test suite so let me add this one.

  class Bad {
   public:
      Bad* operator& () { return this; }
      Bad & operator=3D(Bad const &) & =3D default;
  };

  template<typename T>
  void test() {
      static_assert (__has_trivial_copy (T));
      // T () =3D T ();                     // error
      memcpy (&T (), &T (), sizeof(T));   // warning
  }

  int main() {
      test<Bad>();
  }=