[Bug c++/96501] New: [C++11] Should warn when classes only have copy constructor defined

[Bug c++/96501] New: [C++11] Should warn when classes only have copy constructor defined

[nunoplopes at sapo dot pt]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96501

            Bug ID: 96501
           Summary: [C++11] Should warn when classes only have copy
                    constructor defined
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: nunoplopes at sapo dot pt
  Target Milestone: ---

I see a lot of old code that has copy constructors defined, but not move
constructors. This pessimizes code since the definition of the copy constructor
hides the default move constructor.
Would be nice to get a warning for this to recover perf.

```
#include <utility>
using namespace std;

struct foo {
  int a,b,c;
  // expect-warning
  foo(const foo &);
};

foo fn(foo &&f) {
  foo g(move(f));
  return g;
}
```

https://gcc.godbolt.org/z/5r8erG

[Bug c++/96501] [C++11] Should warn when classes only have copy constructor defined

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96501

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Nuno Lopes from comment #0)
> I see a lot of old code that has copy constructors defined, but not move
> constructors. This pessimizes code since the definition of the copy
> constructor hides the default move constructor.

If you need a user-defined copy constructor, a defaulted move constructor
probably isn't going to do the right thing anyway.

[Bug c++/96501] [C++11] Should warn when classes only have copy constructor defined

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96501

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Also, how would users suppress this warning for cases where it's not wanted?

Plenty of classes don't need a move constructor because moving is not more
efficient than copying, but they don't want a defaulted move constructor.

Basically, the code isn't necessarily wrong, and there's no easy way to tell
whether the lack of a move constructor is a problem, or should be changed.

[Bug c++/96501] [C++11] Should warn when classes only have copy constructor defined

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96501

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |DUPLICATE

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
This is a dup, and there's additional discussion in the dup.

*** This bug has been marked as a duplicate of bug 89700 ***