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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Misleading error message in constexpr function when variable is uninitialized"
href="https://bugs.llvm.org/show_bug.cgi?id=52194">52194</a>
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<th>Summary</th>
<td>Misleading error message in constexpr function when variable is uninitialized
</td>
</tr>
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<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Linux
</td>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>C++2b
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>derekbrowncmu@gmail.com
</td>
</tr>
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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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</tr></table>
<p>
<div>
<pre>Minimal example:
constexpr int func(int a)
{
int sum;
sum += a;
return sum;
}
constexpr int val = func(4);
When I compile this on Godbolt using the "x86-64 clang (trunk)" branch and the
-std=c++20 flag I get the following error message:
<source>:7:15: error: constexpr variable 'val' must be initialized by a
constant expression
constexpr int val = func(4);
^ ~~~~~~~
<source>:4:6: note: subexpression not valid in a constant expression
sum += a;
^
<source>:7:21: note: in call to 'func(4)'
constexpr int val = func(4);
^
1 error generated.
This is misleading because the actual problem is that sum was not initialized.
If sum is initialized with sum=0 then there is no error. This problem exists
for other modify and assign operators as well. Using separate assignment and
equality operators does not have this problem. For example:
constexpr int func(int a)
{
int sum;
sum = sum + a;
return sum;
}
constexpr int val = func(4);
Give this error message:
<source>:7:15: error: constexpr variable 'val' must be initialized by a
constant expression
constexpr int val = func(4);
^ ~~~~~~~
<source>:4:8: note: read of uninitialized object is not allowed in a constant
expression
sum = sum + a;
^
<source>:7:21: note: in call to 'func(4)'
constexpr int val = func(4);
^
1 error generated.
Which correctly identifies that the problem is that sum is uninitialized.</pre>
</div>
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