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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 - Ambiguous instantiation while asserting requirement is not a hard error"
href="https://bugs.llvm.org/show_bug.cgi?id=52524">52524</a>
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<th>Summary</th>
<td>Ambiguous instantiation while asserting requirement is not a hard error
</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>Linux
</td>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Keywords</th>
<td>accepts-invalid
</td>
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<th>Severity</th>
<td>enhancement
</td>
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<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>johelegp@gmail.com
</td>
</tr>
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<th>CC</th>
<td>blitzrakete@gmail.com, erik.pilkington@gmail.com, johelegp@gmail.com, llvm-bugs@lists.llvm.org, richard-llvm@metafoo.co.uk
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<pre>This came up at <a href="https://cpplang.slack.com/archives/C21PKDHSL/p1637078846410400">https://cpplang.slack.com/archives/C21PKDHSL/p1637078846410400</a>.
>From reading <a href="https://wg21.link/LWG3541">https://wg21.link/LWG3541</a>, it seems that the lack of a
disambiguating specialization should result in a hard error, which does not
happen. See <a href="https://godbolt.org/z/vMzPnYvr7">https://godbolt.org/z/vMzPnYvr7</a>.
```C++
#include <concepts>
template<class> struct trait { };
template<class T> concept has_type1 = requires { typename T::type1; };
template<class T> concept has_type2 = requires { typename T::type2; };
template<has_type1 T> struct trait<T> { using type =
typename T::type1; };
template<has_type2 T> struct trait<T> { using type =
typename T::type2; };
template<has_type1 T> requires has_type2<T> and std::same_as<typename T::type1,
typename T::type2>
struct trait<T> { using type = typename T::type1; };
template<class T> concept has_trait_type = requires { typename trait<T>::type;
};
struct Y {
using type1 = double;
using type2 = int;
};
static_assert(not has_trait_type<Y>);
```</pre>
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