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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Unsequenced modifications of variable within a constexpr function used in constant expression context not considered ill-formed"
href="https://bugs.llvm.org/show_bug.cgi?id=38420">38420</a>
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<th>Summary</th>
<td>Unsequenced modifications of variable within a constexpr function used in constant expression context not considered ill-formed
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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>All
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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>C++
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>yaghmour.shafik@gmail.com
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<th>CC</th>
<td>dgregor@apple.com, llvm-bugs@lists.llvm.org
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<p>
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<pre>Given
constexpr int f(int x) {return x++ + x++;}
int main() {
constexpr int x = 2;
constexpr int y = f(x);
}
clang does not consider the declaration of y ill-formed but I believe it
should.
x++ + x++
is undefined behavior since we have unsequenced modifications to x. See
[intro.execution]p10 <a href="http://eel.is/c++draft/intro.execution#10">http://eel.is/c++draft/intro.execution#10</a> and this should
be ill-formed in a context requiring a constant expression since undefined
behavior is ill-formed in a constant expression see [expr.cons]p2.6
<a href="http://eel.is/c++draft/expr.const#2.6">http://eel.is/c++draft/expr.const#2.6</a></pre>
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