[cfe-dev] Q on patch for CWG 2352

[Richard Smith via cfe-dev]

On Thu, 5 Mar 2020 at 11:41, Robinson, Paul via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> > Richard Smith made a patch in f041e9ad for CWG2352.
> > As a consequence of this patch, we had an obscure test failure,
> > and it's not clear to me that it's an intentional consequence.
> > So I figured I'd ask here.
> >
> > Reduced test case:
>
> Slightly edited and expanded test case:
>
> bool foo(void * const * const *       &&) { return false; } // new choice
> bool foo(void *       * const * const &)  { return true; }  // old choice
> bool bar() {
>  return foo(reinterpret_cast<void***>(2));
> }
>

I'm not entirely sure how we were coming up with the old answer, but the
new answer is correct: binding an rvalue reference to an rvalue is
preferred over binding an lvalue reference.


> // A couple of additional cases:
> void*** p;
> void*** q() { return p; }
> bool func1() { return foo(p); } // old and new both call '&' overload
> bool func2() { return foo(q()); } // old calls '&', new calls '&&'
>

Here, q() is an rvalue, so we prefer binding an rvalue reference. p is an
lvalue, so that rule doesn't apply and we prefer binding the lvalue
reference because it binds to a less-qualified type.

I believe the new Clang behavior is following the rules described in C++
[over.ics.rank] paragraph 3.


> So it looks like it matters whether the argument is an expression?
> --paulr
>
>
> > Prior to the patch, the compiler selected the second overload;
> > after the patch, it selects the first overload.  Apparently there's
> > some subtle difference in the preferred-ness of one over the other,
> > and AFAICT the const-nesses aren't supposed to factor in any more?
> > so it's about the &-ref versus the &&-ref?
> >
> > As I said, mainly I want to make sure this was intentional; if it
> > is, we can fiddle our test and that's the end of it.  But if it's
> > not intentional, this change is in the almost-final Clang 10.0
> > release, and might want to be fixed before it goes out.
> >
> > Thanks,
> > --paulr
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