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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - -Wconstant-evaluated erroneously claims an is_constant_evaluated() will always evaluate as true"
href="https://bugs.llvm.org/show_bug.cgi?id=44415">44415</a>
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<th>Summary</th>
<td>-Wconstant-evaluated erroneously claims an is_constant_evaluated() will always evaluate as true
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<th>Product</th>
<td>clang
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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
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>C++2a
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>tonyelewis@hotmail.com
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<th>CC</th>
<td>blitzrakete@gmail.com, erik.pilkington@gmail.com, llvm-bugs@lists.llvm.org, richard-llvm@metafoo.co.uk
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<pre>Thanks for the work to emit a warning on `if constexpr
(std::is_constant_evaluated())` (<a class="bz_bug_link
bz_status_RESOLVED bz_closed"
title="RESOLVED FIXED - Please warn on if constexpr (std::is_constant_evaluated())"
href="show_bug.cgi?id=42977">https://bugs.llvm.org/show_bug.cgi?id=42977</a>),
which I think is a great idea. Here's a wrinkle...
Using Godbolt's clang trunk (31st December 2019,
6185dc0eb3ad35e1f85f2ab1038ca978563099f4) with `-std=c++2a -O1` to compile the
following:
#include <type_traits>
int main() {
int b = 2;
const int a = std::is_constant_evaluated() ? b : 1;
return a;
}
I get this warning:
<source>:5:19: warning: 'std::is_constant_evaluated' will always evaluate to
'true' in a manifestly constant-evaluated expression [-Wconstant-evaluated]
const int a = std::is_constant_evaluated() ? b : 1;
^
...yet the generated code is returning 1, showing that the
`is_constant_evaluated()` has evaluated to false and thus that the warning is
incorrect.
The example originates from a snippet in Daveed Vandevoorde's "C++ Constants"
C++Now 2019 talk (around 39m00s).</pre>
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