[Bug c++/113219] New: Overloaded ref-qualified conversion operator triggers bogus -Wconversion

[Bug c++/113219] New: Overloaded ref-qualified conversion operator triggers bogus -Wconversion

[m.cencora at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113219

            Bug ID: 113219
           Summary: Overloaded ref-qualified conversion operator triggers
                    bogus -Wconversion
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: m.cencora at gmail dot com
  Target Milestone: ---

Following code when compiled on any gcc version (checked all available on
godbolt since gcc-7) triggers conversion warnings.
I consider it bogus because it tells user that it has chosen a better overload
(which is good thing, right?).
Clang doesn't print any related warning even with -Weverything

g++ -std=c++17 -Wconversion

template <typename T>
struct wrapper
{
    operator T() const&;

    operator T&&() &&;
};

wrapper<int> createInt();

int test()
{
    return createInt();
}

Output is:
<source>:13:21: warning: choosing 'wrapper<T>::operator T&&() && [with T =
int]' over 'wrapper<T>::operator T() const & [with T = int]' [-Wconversion]
   13 |     return createInt();
      |            ~~~~~~~~~^~
<source>:13:21: warning:   for conversion from 'wrapper<int>' to 'int'
[-Wconversion]
<source>:13:21: note:   because conversion sequence for the argument is better

[Bug c++/113219] Overloaded ref-qualified conversion operator triggers bogus -Wconversion

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113219

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

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

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
This is documented this way too:
https://gcc.gnu.org/onlinedocs/gcc-13.2.0/gcc/Warning-Options.html#index-Wconversion
```
For C++, also warn for confusing overload resolution for user-defined
conversions; and conversions that never use a type conversion operator:
conversions to void, the same type, a base class or a reference to them.
Warnings about conversions between signed and unsigned integers are disabled by
default in C++ unless -Wsign-conversion is explicitly enabled.

```


Don't use -Wconversion if you don't wan this warning ...

Note this warning was originally not part of -Wconversion but always on (in GCC
2.95.3) and then in GCC 3.4.0 was moved to part of -Wconversion; this was PR
10784 was about even.

[Bug c++/113219] Overloaded ref-qualified conversion operator triggers bogus -Wconversion

[m.cencora at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113219

--- Comment #2 from m.cencora at gmail dot com ---
So I guess this falls into the "confusing overload resolution for user-defined
conversion" but I fail to see what can be confusing here.
createInt() returns prvalue, so later it binds to xvalue for the purpose of
implicit conversion operator invocation. That means that r-value qualified
conversion operator is chosen.

If your advice is to avoid using "-Wconversion" that it is rather unfortunate
because user looses good warnings (e.g. about conversions that may alter the
value).
Also there is no way to silence the warning for such a code, since gcc warns on
each call-site, not on definition-site of the overloads.
So it either unfeasible or even impossible if the definitions are in library
code, but warnings are enabled in user code.

Please consider removing the warning generation in such scenarios.