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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Clang-CL's MSVC preprocessor emulation does not properly handle variadic arguments"
href="https://bugs.llvm.org/show_bug.cgi?id=45005">45005</a>
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<th>Summary</th>
<td>Clang-CL's MSVC preprocessor emulation does not properly handle variadic arguments
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<th>Product</th>
<td>clang
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<th>Version</th>
<td>9.0
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<th>Hardware</th>
<td>PC
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<th>OS</th>
<td>Windows NT
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<th>Status</th>
<td>NEW
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<th>Severity</th>
<td>normal
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>C++
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>Balazs.CZIRAKI@nng.com
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<th>CC</th>
<td>blitzrakete@gmail.com, dgregor@apple.com, erik.pilkington@gmail.com, llvm-bugs@lists.llvm.org, richard-llvm@metafoo.co.uk
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<pre>Clang-CL's MSVC preprocessor emulation does not produce the exact same behavior
as MSVC's preprocessor, which breaks certain libraries that are prepared to
handle MSVC's preprocessor behavior.
A minimal repro of the error that causes said libraries to break:
<a href="https://godbolt.org/z/c5RndM">https://godbolt.org/z/c5RndM</a>
(Clang-CL is not available on Godbolt, so the output of a local build was
pasted into a comment.)
The example macro `_NARG` counts the arguments it was provided up to three.
Since MSVC's preprocessor has it's quirks, the `_NARG_MSVC` macro is what can
produce the same behavior with that preprocessor.
When given two empty arguments, Clang-CL's preprocessor does not properly
handle the MSVC version, and produces an erroneous argument count of `1`.
This breaks libraries such as:
* GoogleMock: <a href="https://godbolt.org/z/noqbnj">https://godbolt.org/z/noqbnj</a>
(The new `MOCK_METHOD` macro in 1.10)
* Boost Preprocessor: <a href="https://godbolt.org/z/NL6BbA">https://godbolt.org/z/NL6BbA</a></pre>
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