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<p class="MsoNormal">+Hans for 10.0 visibility.<o:p></o:p></p>
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<p class="MsoNormal"><b><span style="font-size:12.0pt;color:black">From: </span></b><span style="font-size:12.0pt;color:black">Richard Smith <richard@metafoo.co.uk><br>
<b>Reply-To: </b>"richard@metafoo.co.uk" <richard@metafoo.co.uk><br>
<b>Date: </b>Friday, March 6, 2020 at 4:28 PM<br>
<b>To: </b>"Robinson, Paul" <paul.robinson@sony.com><br>
<b>Cc: </b>Shoaib Meenai <smeenai@fb.com>, David Blaikie <dblaikie@gmail.com>, cfe-dev <cfe-dev@lists.llvm.org><br>
<b>Subject: </b>Re: [cfe-dev] Q on patch for CWG 2352<o:p></o:p></span></p>
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<p class="MsoNormal">On Thu, 5 Mar 2020 at 11:41, Robinson, Paul via cfe-dev <<a href="mailto:cfe-dev@lists.llvm.org">cfe-dev@lists.llvm.org</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal">> Richard Smith made a patch in f041e9ad for CWG2352.<br>
> As a consequence of this patch, we had an obscure test failure,<br>
> and it's not clear to me that it's an intentional consequence.<br>
> So I figured I'd ask here.<br>
><br>
> Reduced test case:<br>
<br>
Slightly edited and expanded test case:<br>
<br>
bool foo(void * const * const * &&) { return false; } // new choice<br>
bool foo(void * * const * const &) { return true; } // old choice<br>
bool bar() {<br>
return foo(reinterpret_cast<void***>(2));<br>
}<o:p></o:p></p>
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<p class="MsoNormal">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.<o:p></o:p></p>
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<p class="MsoNormal">// A couple of additional cases:<br>
void*** p;<br>
void*** q() { return p; }<br>
bool func1() { return foo(p); } // old and new both call '&' overload<br>
bool func2() { return foo(q()); } // old calls '&', new calls '&&'<o:p></o:p></p>
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<p class="MsoNormal">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.<o:p></o:p></p>
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<p class="MsoNormal">I believe the new Clang behavior is following the rules described in C++ [over.ics.rank] paragraph 3.<o:p></o:p></p>
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<p class="MsoNormal">So it looks like it matters whether the argument is an expression?<br>
--paulr<br>
<br>
<br>
> Prior to the patch, the compiler selected the second overload;<br>
> after the patch, it selects the first overload. Apparently there's<br>
> some subtle difference in the preferred-ness of one over the other,<br>
> and AFAICT the const-nesses aren't supposed to factor in any more?<br>
> so it's about the &-ref versus the &&-ref?<br>
><br>
> As I said, mainly I want to make sure this was intentional; if it<br>
> is, we can fiddle our test and that's the end of it. But if it's<br>
> not intentional, this change is in the almost-final Clang 10.0<br>
> release, and might want to be fixed before it goes out.<br>
><br>
> Thanks,<br>
> --paulr<br>
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