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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">For something to be a constant expression, it has to (going by C++17 section 8.20 [expr.const])<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">* be a core constant expression, which includes all of the subexpressions being core constant expressions (see paragraph 2)<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">* satisfy certain constraints in paragraph 5<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Here the expression x isn’t a core constant expression because x isn’t constexpr (see paragraph 2.7.2).<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">(GCC will actually give an error here if the pointer argument of g is dereferenced.)<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">However even if you declare x as constexpr we still get an error. I think this part is a bug in clang – if<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">x is declared constexpr then the x in g(x) isn’t a constant expression (according to paragraph 5.2 it has<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">to have static storage duration) but that doesn’t matter because g(x) is. Or in other words, I think that<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">for a constexpr function call to be an actual constant expression it has to have arguments that are core<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">constant expressions, but they do not need to be constant expressions.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">John<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> cfe-dev [mailto:cfe-dev-bounces@lists.llvm.org]
<b>On Behalf Of </b>Anna Welker via cfe-dev<br>
<b>Sent:</b> 17 October 2019 18:01<br>
<b>To:</b> via cfe-dev<br>
<b>Subject:</b> [cfe-dev] [RFD] Are enums allowed to capture local const variables?<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:black">Hi all,<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:black"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:black">The following code snippet will compile with GCC, but not with LLVM:<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:black"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:black">constexpr unsigned g(const float *) { return 3; }<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:black">unsigned f() {<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:black"> const float x[] = {1.0};<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:black"> enum { SIZE = g(x) };<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:black"> return SIZE;<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:black">}<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:black"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:black">At first glance, this looks fine to me, as the expression in the assignment is a constexpr function call with a const parameter.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:black">Clang however errors with "reference to local variable 'x' declared in enclosing function".<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:black"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:black">I traced this down to the function getParentOfCapturingContextOrNull in SemaExpr.cpp, which will diagnose the variable reference uncapturable if it does not appear in a block
literal, captured statement, or lambda expression.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:black"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:black">My question is: Why does this list not contain enum declarations, at least if the declaration is local and the variable is in the same scope?<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:black"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:black">I tried to find a mention of this in the C++ standard, but did not succeed so far.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:black">If this is a bug I'm happy to file it on bugzilla, but I first wanted to ask what the opinions are on whether this code should be valid or not.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:black"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:black">Kind regards,<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:black">Anna<o:p></o:p></span></p>
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