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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Improper handling of __declspec(selectany) constexpr in clang-cl /std:c++17"
href="https://bugs.llvm.org/show_bug.cgi?id=43270">43270</a>
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<th>Summary</th>
<td>Improper handling of __declspec(selectany) constexpr in clang-cl /std:c++17
</td>
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<th>Product</th>
<td>clang
</td>
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Linux
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>enhancement
</td>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>-New Bugs
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>epastor@google.com
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<th>CC</th>
<td>htmldeveloper@gmail.com, llvm-bugs@lists.llvm.org, neeilans@live.com, richard-llvm@metafoo.co.uk
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<pre>Created <span class=""><a href="attachment.cgi?id=22484" name="attach_22484" title="Minimal reproduction">attachment 22484</a> <a href="attachment.cgi?id=22484&action=edit" title="Minimal reproduction">[details]</a></span>
Minimal reproduction
For MSVC compatibility, clang-cl replicates a non-standard behavior creating
implicit definitions for static const (including constexpr) members.
According to Microsoft documentation, to accommodate these, we're supposed to
use __declspec(selectany) as a guard on the out-of-class definition. [1]
However... if we pass /std:c++17 or /std:c++2a to clang-cl when compiling a
source file including an out-of-class definition, clang-cl emits the following
warning:
=====
selectany.cc(13,1): warning: attribute declaration must precede definition
[-Wignored-attributes]
SELECTANY constexpr int CMyClass::val;
^
selectany.cc(8,30): note: expanded from macro 'SELECTANY'
#define SELECTANY __declspec(selectany)
^
selectany.cc(3,24): note: previous definition is here
static constexpr int val = 0;
=====
No such warning appears if we pass /std:c++14 instead.
Minimal reproduction attached.
[1]:
<a href="https://docs.microsoft.com/en-us/cpp/build/reference/microsoft-extensions-to-c-and-cpp?view=vs-2019">https://docs.microsoft.com/en-us/cpp/build/reference/microsoft-extensions-to-c-and-cpp?view=vs-2019</a></pre>
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