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Right. I was just bringing Richard's reply to Hans' attention, to make sure he saw that the new behavior was correct and there were no concerns here for the 10.0 release. </div>
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<div id="divRplyFwdMsg" dir="ltr" data-ogsc="" style=""><font face="Calibri, sans-serif" color="#000000" style="font-size: 11pt;" data-ogsc=""><b>From:</b> Robinson, Paul <paul.robinson@sony.com><br>
<b>Sent:</b> Monday, March 9, 2020 5:38:45 AM<br>
<b>To:</b> Shoaib Meenai <smeenai@fb.com>; richard@metafoo.co.uk <richard@metafoo.co.uk><br>
<b>Cc:</b> David Blaikie <dblaikie@gmail.com>; cfe-dev@lists.llvm.org <cfe-dev@lists.llvm.org>; Hans Wennborg <hans@chromium.org><br>
<b>Subject:</b> RE: [cfe-dev] Q on patch for CWG 2352</font>
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<p class="x_MsoNormal">> +Hans for 10.0 visibility.</p>
<p class="x_MsoNormal"> </p>
<p class="x_MsoNormal">The new (corrected) behavior is in 10.0, there is nothing to fix here. Arguably there could be a release note.</p>
<p class="x_MsoNormal">--paulr</p>
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<p class="x_MsoNormal"><b>From:</b> Shoaib Meenai <smeenai@fb.com> <br>
<b>Sent:</b> Friday, March 6, 2020 7:35 PM<br>
<b>To:</b> richard@metafoo.co.uk; Robinson, Paul <paul.robinson@sony.com><br>
<b>Cc:</b> David Blaikie <dblaikie@gmail.com>; cfe-dev@lists.llvm.org; Hans Wennborg <hans@chromium.org><br>
<b>Subject:</b> Re: [cfe-dev] Q on patch for CWG 2352</p>
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<p class="x_MsoNormal">+Hans for 10.0 visibility.</p>
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<p class="x_MsoNormal"><b><span style="font-size: 12pt; color: black;">From: </span>
</b><span style="font-size: 12pt; color: black;">Richard Smith <<a href="mailto:richard@metafoo.co.uk" data-ogsc="" style="">richard@metafoo.co.uk</a>><br>
<b>Reply-To: </b>"<a href="mailto:richard@metafoo.co.uk" data-ogsc="" style="">richard@metafoo.co.uk</a>" <<a href="mailto:richard@metafoo.co.uk" data-ogsc="" style="">richard@metafoo.co.uk</a>><br>
<b>Date: </b>Friday, March 6, 2020 at 4:28 PM<br>
<b>To: </b>"Robinson, Paul" <<a href="mailto:paul.robinson@sony.com" data-ogsc="" style="">paul.robinson@sony.com</a>><br>
<b>Cc: </b>Shoaib Meenai <<a href="mailto:smeenai@fb.com" data-ogsc="" style="">smeenai@fb.com</a>>, David Blaikie <<a href="mailto:dblaikie@gmail.com" data-ogsc="" style="">dblaikie@gmail.com</a>>, cfe-dev <<a href="mailto:cfe-dev@lists.llvm.org" data-ogsc="" style="">cfe-dev@lists.llvm.org</a>><br>
<b>Subject: </b>Re: [cfe-dev] Q on patch for CWG 2352</span></p>
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<p class="x_MsoNormal">On Thu, 5 Mar 2020 at 11:41, Robinson, Paul via cfe-dev <<a href="mailto:cfe-dev@lists.llvm.org" data-ogsc="" style="">cfe-dev@lists.llvm.org</a>> wrote:</p>
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<p class="x_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>
}</p>
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<p class="x_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.</p>
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<p class="x_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 '&&'</p>
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<p class="x_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.</p>
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<p class="x_MsoNormal">I believe the new Clang behavior is following the rules described in C++ [over.ics.rank] paragraph 3.</p>
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<p class="x_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>
_______________________________________________<br>
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<div id="divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" style="font-size:11pt" color="#000000"><b>From:</b> Robinson, Paul <paul.robinson@sony.com><br>
<b>Sent:</b> Monday, March 9, 2020 5:38:45 AM<br>
<b>To:</b> Shoaib Meenai <smeenai@fb.com>; richard@metafoo.co.uk <richard@metafoo.co.uk><br>
<b>Cc:</b> David Blaikie <dblaikie@gmail.com>; cfe-dev@lists.llvm.org <cfe-dev@lists.llvm.org>; Hans Wennborg <hans@chromium.org><br>
<b>Subject:</b> RE: [cfe-dev] Q on patch for CWG 2352</font>
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<p class="x_MsoNormal">> +Hans for 10.0 visibility.</p>
<p class="x_MsoNormal"> </p>
<p class="x_MsoNormal">The new (corrected) behavior is in 10.0, there is nothing to fix here. Arguably there could be a release note.</p>
<p class="x_MsoNormal">--paulr</p>
<p class="x_MsoNormal"> </p>
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<p class="x_MsoNormal"><b>From:</b> Shoaib Meenai <smeenai@fb.com> <br>
<b>Sent:</b> Friday, March 6, 2020 7:35 PM<br>
<b>To:</b> richard@metafoo.co.uk; Robinson, Paul <paul.robinson@sony.com><br>
<b>Cc:</b> David Blaikie <dblaikie@gmail.com>; cfe-dev@lists.llvm.org; Hans Wennborg <hans@chromium.org><br>
<b>Subject:</b> Re: [cfe-dev] Q on patch for CWG 2352</p>
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<p class="x_MsoNormal"> </p>
<p class="x_MsoNormal">+Hans for 10.0 visibility.</p>
<p class="x_MsoNormal"> </p>
<div style="border:none; border-top:solid #B5C4DF 1.0pt; padding:3.0pt 0in 0in 0in">
<p class="x_MsoNormal"><b><span style="font-size:12.0pt; color:black">From: </span>
</b><span style="font-size:12.0pt; color:black">Richard Smith <<a href="mailto:richard@metafoo.co.uk">richard@metafoo.co.uk</a>><br>
<b>Reply-To: </b>"<a href="mailto:richard@metafoo.co.uk">richard@metafoo.co.uk</a>" <<a href="mailto:richard@metafoo.co.uk">richard@metafoo.co.uk</a>><br>
<b>Date: </b>Friday, March 6, 2020 at 4:28 PM<br>
<b>To: </b>"Robinson, Paul" <<a href="mailto:paul.robinson@sony.com">paul.robinson@sony.com</a>><br>
<b>Cc: </b>Shoaib Meenai <<a href="mailto:smeenai@fb.com">smeenai@fb.com</a>>, David Blaikie <<a href="mailto:dblaikie@gmail.com">dblaikie@gmail.com</a>>, cfe-dev <<a href="mailto:cfe-dev@lists.llvm.org">cfe-dev@lists.llvm.org</a>><br>
<b>Subject: </b>Re: [cfe-dev] Q on patch for CWG 2352</span></p>
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<p class="x_MsoNormal"> </p>
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<p class="x_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:</p>
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<p class="x_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>
}</p>
</blockquote>
<div>
<p class="x_MsoNormal"> </p>
</div>
<div>
<p class="x_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.</p>
</div>
<div>
<p class="x_MsoNormal"> </p>
</div>
<blockquote style="border:none; border-left:solid #CCCCCC 1.0pt; padding:0in 0in 0in 6.0pt; margin-left:4.8pt; margin-top:5.0pt; margin-right:0in; margin-bottom:5.0pt">
<p class="x_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 '&&'</p>
</blockquote>
<div>
<p class="x_MsoNormal"> </p>
</div>
<div>
<p class="x_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.</p>
</div>
<div>
<p class="x_MsoNormal"> </p>
</div>
<div>
<p class="x_MsoNormal">I believe the new Clang behavior is following the rules described in C++ [over.ics.rank] paragraph 3.</p>
</div>
<div>
<p class="x_MsoNormal"> </p>
</div>
<blockquote style="border:none; border-left:solid #CCCCCC 1.0pt; padding:0in 0in 0in 6.0pt; margin-left:4.8pt; margin-top:5.0pt; margin-right:0in; margin-bottom:5.0pt">
<p class="x_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>
_______________________________________________<br>
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<a href="mailto:cfe-dev@lists.llvm.org" target="_blank">cfe-dev@lists.llvm.org</a><br>
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