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<p class="MsoNormal"><span lang="EN-US" style="mso-fareast-language:EN-US">I tracked this down to be dependent on the MSVC Conformance mode (“/permissive-“ flag [1], which explicitly disables certain non-conforming behavior that MSVC would accept otherwise).
See the following minimal example: <a href="https://godbolt.org/z/fBaWdQ">https://godbolt.org/z/fBaWdQ</a>
<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="mso-fareast-language:EN-US">In conforming mode, MSVC reports the ambiguity, in permissive mode it doesn’t. Clang, gcc and icc all accept the code. I *<b>think</b>* MSVC is wrong in reporting the ambiguity, since
unqualified lookup inside the definition of N2::N3::f – even though f is defined outside N2::N3 – should happen scope by scope for N3, N2 and finally global. S is visible for the first time in N2 (through “using N1::S”). However, the allegedly ambiguous alias
template S is only introduced in the global scope, which should not be searched at all, since we already found S in N2.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="mso-fareast-language:EN-US">Moreover, it is interesting to note that if C is not a class template, MSVC accepts the code even in conforming mode:
<a href="https://godbolt.org/z/O7jbq2">https://godbolt.org/z/O7jbq2</a> <o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="mso-fareast-language:EN-US">If my reasoning above is correct, this is a bug in MSVC under conforming mode.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="mso-fareast-language:EN-US">Reid, thank you for your patch in 19f1dc7. Note that directly including clang/Basic/LLVM.h is not necessary, since it is already included indirectly via ByteCodeStmtGen.h. Moreover,
the same alias templates are also present in ByteCodeExprGen.cpp, you might want to remove them there as well.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="mso-fareast-language:EN-US">[1]: <a href="https://docs.microsoft.com/en-us/cpp/build/reference/permissive-standards-conformance?view=vs-2019">
https://docs.microsoft.com/en-us/cpp/build/reference/permissive-standards-conformance?view=vs-2019</a>
<o:p></o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US">From:</span></b><span lang="EN-US"> Reid Kleckner <rnk@google.com>
<br>
<b>Sent:</b> Donnerstag, 31. Oktober 2019 22:32<br>
<b>To:</b> Jonathan Meier <jonathan.meier@outlook.com><br>
<b>Cc:</b> Osman Zakir <osmanzakir90@hotmail.com>; llvm-dev@lists.llvm.org<br>
<b>Subject:</b> Re: [llvm-dev] Recent LLVM Build Attempt -- obj.ClangAST.vcxproj Failed to Build<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US">I went ahead and did that in 19f1dc7. I wasn't able to reproduce the problem locally, but I am not building through Visual Studio, I am using ninja. To build in C++17 mode, I just re-ran with -std:c++17, and got warnings
but no errors.<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US">On Thu, Oct 31, 2019 at 11:11 AM Jonathan Meier <</span><a href="mailto:jonathan.meier@outlook.com"><span lang="EN-US">jonathan.meier@outlook.com</span></a><span lang="EN-US">> wrote:<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US">From a quick glance at the build log, you should be able to avoid the ambiguity by removing the alias template
<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US">`</span><span lang="EN-US" style="font-size:9.0pt;font-family:Consolas;color:#D73A49">template
</span><span lang="EN-US" style="font-size:9.0pt;font-family:Consolas;color:#24292E;background:white"><</span><span lang="EN-US" style="font-size:9.0pt;font-family:Consolas;color:#D73A49">typename</span><span lang="EN-US" style="font-size:9.0pt;font-family:Consolas;color:#24292E;background:white">
T> </span><span lang="EN-US" style="font-size:9.0pt;font-family:Consolas;color:#D73A49">using</span><span lang="EN-US" style="font-size:9.0pt;font-family:Consolas;color:#24292E;background:white"> Optional = llvm::Optional<T>;`</span><span lang="EN-US"><o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US">from the files ByteCodeExprGen.cpp and ByteCodeStmtGen.cpp in clang/lib/AST/Interp. <o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US">Note that this is just my best guess, I didn‘t try it myself. I‘m also not sure why MSVC sees an ambiguity here. <o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
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<o:p></o:p></span></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt">Am 31.10.2019 um 14:44 schrieb Osman Zakir via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a>>:<o:p></o:p></p>
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<p class="MsoNormal"> <span lang="EN-US"><o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt;color:black">I hope I'm not being too presumptuous or anything when I say this: could you guys please review the obj.ClangAST.vcxproj project and try to fix the ambiguity error? <o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt;color:black"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt;color:black">Reid asked me why I'm using C++17. I just want to use the latest released standard. Most if it still compiles anyway. So I'll stick with it. <o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US">_______________________________________________<br>
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