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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Fold-expression doesn't properly look up operator"
href="https://bugs.llvm.org/show_bug.cgi?id=42518">42518</a>
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<th>Summary</th>
<td>Fold-expression doesn't properly look up operator
</td>
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<th>Product</th>
<td>clang
</td>
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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>Windows NT
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>enhancement
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>C++17
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>barry.revzin@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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<p>
<div>
<pre>From StackOverflow (<a href="https://stackoverflow.com/q/56898075/2069064">https://stackoverflow.com/q/56898075/2069064</a>):
enum Enum {
A = 3,
B = 8,
C = 5
};
namespace EnumMax {
constexpr Enum operator>>=(const Enum left, const Enum right) {
return left < right ? right : left;
}
}
template<Enum ... enums>
constexpr Enum max() {
using EnumMax::operator>>=;
return (enums >>= ...);
}
constexpr Enum max_v = max<A, B, C>();
The fold-expression (enums >>= ...) does not find the operator>>= brought in
with the using-declaration. The error reported is:
<source>:16:23: error: expression is not assignable
return (enums >>= ...);
~~~~~ ^
The same is true if enums is a function parameter pack instead of a template
non-type parameter pack.
Manually expanding the pack (as the OP in the question demonstrates) does
compile:
template<Enum e1, Enum e2, Enum e3>
constexpr Enum max() {
using EnumMax::operator>>=;
return (e1 >>= (e2 >>= e3));
}</pre>
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