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<base href="https://bugs.llvm.org/">
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<body><table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - constexpr variable cannot be implicitly captured in C++17 lambda expression"
href="https://bugs.llvm.org/show_bug.cgi?id=45172">45172</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>constexpr variable cannot be implicitly captured in C++17 lambda expression
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>9.0
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>Linux
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>C++17
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>mali2011@qq.com
</td>
</tr>
<tr>
<th>CC</th>
<td>blitzrakete@gmail.com, erik.pilkington@gmail.com, llvm-bugs@lists.llvm.org, richard-llvm@metafoo.co.uk
</td>
</tr></table>
<p>
<div>
<pre>// C++ code
static bool lessThan(const float& x, const float& value)
{
return x < value;
}
int main()
{
constexpr float THRESHOLD = 1E-6;
auto compare1 = [](const float& lhs, const float& rhs)
{
return (lhs - rhs) <= THRESHOLD;
};
auto compare2 = [](const float& lhs, const float& rhs)
{
return lessThan(lhs - rhs, THRESHOLD);
};
auto compare3 = [](const float& lhs, const float& rhs)
{
float THRESHOLD2 = THRESHOLD;
return (lhs - rhs) <= THRESHOLD2;
};
float a = 1.0F, b = 2.0F;
return compare1(a, b) && compare2(a, b) && compare3(a, b);
}
/************************************************************
when I use clang 6 or clang 9 compiler with C++17 enabled, it complains with
error pasted below. GCC is happy with this. And constexpr variable doesn't need
to capture in C++17 standard.
compare1 passes, compare2 fails, compare3 passes. That's a weird result, When I
move the constexpr variable out of static function call parameter to a local
variable(compare2 => compare3), the compiler doesn't complain anymore.
-------------------------------------------------------------
martin@M2037:~/Desktop/$ clang++ test.cpp -o test -Wall -std=c++17
test.cpp:13:30: error: variable 'THRESHOLD' cannot be implicitly captured in a
lambda with no capture-default specified
return lessThan(lhs - rhs, THRESHOLD);
^
test.cpp:9:18: note: 'THRESHOLD' declared here
constexpr float THRESHOLD = 1E-6;
^
test.cpp:11:18: note: lambda expression begins here
auto compare1 = [](const float& lhs, const float& rhs)
^
1 error generated.
martin@M2037:~/Desktop/$ clang -v
clang version 9.0.0-2~ubuntu18.04.2 (tags/RELEASE_900/final)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
Found candidate GCC installation: /usr/bin/../lib/gcc/i686-linux-gnu/8
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/7
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/7.4.0
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/8
Found candidate GCC installation: /usr/lib/gcc/i686-linux-gnu/8
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/7
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/7.4.0
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/8
Selected GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/8
Candidate multilib: .;@m64
Candidate multilib: 32;@m32
Candidate multilib: x32;@mx32
Selected multilib: .;@m64
************************************************************/</pre>
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