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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Premature substitution into constraints"
href="https://bugs.llvm.org/show_bug.cgi?id=47414">47414</a>
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<th>Summary</th>
<td>Premature substitution into constraints
</td>
</tr>
<tr>
<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>
</tr>
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<th>OS</th>
<td>Windows NT
</td>
</tr>
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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>
</tr>
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<th>Component</th>
<td>C++2a
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>Casey@Carter.net
</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
</td>
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<p>
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<pre>Compiling this program:
template <class, class> constexpr bool is_same_v = false;
template <class T> constexpr bool is_same_v<T, T> = true;
template <class T> concept C = is_same_v<T, void>;
template <class T> requires C<T> using alias = T;
template <class T> struct S {
void f() requires C<T> && is_same_v<alias<T>, void> {}
};
int main() { S<int>{}; }
with "clang++ -std=c++2a" diagnoses (<a href="https://godbolt.org/z/qhhec8">https://godbolt.org/z/qhhec8</a>):
<source>:9:41: error: constraints not satisfied for alias template 'alias'
[with T = int]
void f() requires C<T> && is_same_v<alias<T>, void> {}
^~~~~~~~
<source>:12:14: note: in instantiation of template class 'S<int>' requested
here
int main() { S<int>{}; }
^
<source>:6:29: note: because 'int' does not satisfy 'C'
template <class T> requires C<T> using alias = T;
^
<source>:4:32: note: because 'is_same_v<int, void>' evaluated to false
template <class T> concept C = is_same_v<T, void>;
^
Despite that `C<T>` is unsatisfied, the compiler is substituting into the
conjoined `is_same_v<alias<T>, void>` atomic constraint.</pre>
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