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<p>FWIW, <span style="font-size: 12pt;">I've performed an experiment with the code below at godbolt.</span></p>
<div>(used -O2, https://godbolt.org/z/nY95nh)<br>
</div>
<div><br>
</div>
<div>```</div>
<div>#include <vector></div>
<div>#include "llvm/Support/Error.h"</div>
<div><br>
</div>
<div>llvm::Expected<std::vector<int>> foo() {</div>
<div> std::vector<int> V;</div>
<div> V.push_back(0);</div>
<div> return V;</div>
<div>}</div>
<div>```<br>
</div>
<div><br>
</div>
<div>If I understand the produced output correctly, then results are:</div>
<div><br>
</div>
<div>gcc 7.5: creates a copy.</div>
<div>gcc 8.1: uses move.</div>
<div><br>
</div>
<div>clang < 6.0: doesn't compile.</div>
<div>clang >= 6.0: uses move.</div>
<div><br>
</div>
<div>MSVS: was unable to compile, complains about "llvm/Support/Error.h" header.</div>
<div>I am using MSVS 2017 locally and it calls move constructor of Expected<> though,</div>
<div>so I think all MSVS >= 2017 (at least) should be fine.<br>
</div>
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<div class="PlainText">Best regards,<br>
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<div id="divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" color="#000000" style="font-size:11pt"><b>От:</b> Alexander Shaposhnikov <alexander.v.shaposhnikov@gmail.com><br>
<b>Отправлено:</b> 28 сентября 2020 г. 22:46<br>
<b>Кому:</b> David Blaikie<br>
<b>Копия:</b> Richard Smith; llvm-dev; Lang Hames; George Rimar; James Henderson; avl.lapshin@gmail.com<br>
<b>Тема:</b> [EXTERNAL] Re: [llvm-dev] preferred way to return expected values</font>
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<div>
<div dir="ltr">Many thanks for the reply,
<div>right, this is what the discussion is about.</div>
<div><br>
</div>
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<br>
<div class="gmail_quote">
<div dir="ltr" class="gmail_attr">On Mon, Sep 28, 2020 at 10:57 AM David Blaikie <<a href="mailto:dblaikie@gmail.com">dblaikie@gmail.com</a>> wrote:<br>
</div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex; border-left:1px solid rgb(204,204,204); padding-left:1ex">
<div dir="ltr">To clarify, this is a discussion around whether given some move-only type X, implicitly convertible to Y and the code "Y func() { X x; return x; }" is that valid in LLVM? (and, as a corollary, if the type isn't move-only, is that code efficient
(does it move rather than copying) - as in the vector example given)<br>
<br>
I /believe/ the answer is that it is not valid. I think the set of compilers supported includes those that do not implement this rule. (either due to the language version we compile with, or due to it being a DR that some supported compiler versions do not
implement). But that's just my rough guess.</div>
<br>
<div class="gmail_quote">
<div dir="ltr" class="gmail_attr">On Sat, Sep 26, 2020 at 3:17 PM Alexander Shaposhnikov via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a>> wrote:<br>
</div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex; border-left:1px solid rgb(204,204,204); padding-left:1ex">
<div dir="ltr">Hello everyone!
<div>
<div>
<div>It looks like in the LLVM codebase (including subprojects) there are some inconsistencies <br>
<div>in how values are returned from functions with the following (or similar) signature:</div>
<div> Expected<std::vector<int>> createVector() {</div>
<div> std::vector<int> V;</div>
<div> ... </div>
<div> }</div>
<div>It would be interesting to find out your opinion on this.</div>
<div>After some investigation I have found <a href="https://reviews.llvm.org/D70963" target="_blank">https://reviews.llvm.org/D70963</a> and
<a href="https://reviews.llvm.org/D43322" target="_blank">https://reviews.llvm.org/D43322</a> which have some additional context regarding </div>
</div>
<div>the problem. The aforementioned diffs (and the comments on them) contain a lot of</div>
</div>
</div>
<div>details and the history of the problem (whether one should use the cast or not).</div>
<div>If I am not mistaken a part of the problem is that compilers' behaviors have changed over time, and e.g. the latest versions would use a move constructor while the older ones could use a copy constructor. So the question is where we stand at the moment
/ what is the recommended approach for the new code.</div>
<div><br>
</div>
<div>Many thanks in advance,</div>
<div>Alexander Shaposhnikov</div>
<div><br>
</div>
</div>
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