Re: [PATCH v2] c++: Extend -Wredundant-move for const-qual objects [PR90428]

[Marek Polacek <polacek@redhat.com>]

On Mon, Aug 15, 2022 at 03:43:38PM -0400, Jason Merrill wrote:
> On 8/8/22 13:27, Marek Polacek wrote:
> > This is to warn about this:
> > 
> > T f5(const T t)
> > {
> >    return std::move(t); // { dg-warning "redundant move" }
> > }
> > 
> > where OR fails because there's no T(const T&&) (or it's deleted in which case
> > convert_for_initialization also returns error_mark_node).  This OR is going to
> > fail with std::move but also without std::move when we're trying to treat an lvalue
> > as an rvalue.  So the std::move has no effect, because T(const T&) will be
> > called in either case.
> > 
> > Now, if there was a T(const T&&), we'd *still* warn, because the std::move would
> > still be redundant.  Does that make sense?
> 
> Ah, so this is because LOOKUP_PREFER_RVALUE returns an error if a const T&
> overload is selected.  The comment should mention that.  OK with that
> change.
 
Thanks, pushed.

> Since you've been thinking about this area, I wonder if you want to look at
> implementing P2266?

Yup, I can take a look.  It's also come up recently in a discussion with Davis
in https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89780#c3.

Since P2266 was approved, I've updated our C++23 status table as well.

Marek