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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 - Weird failure/interaction in substitution/specialization matching"
href="https://bugs.llvm.org/show_bug.cgi?id=38980">38980</a>
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<th>Summary</th>
<td>Weird failure/interaction in substitution/specialization matching
</td>
</tr>
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<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<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>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>C++
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>Casey@Carter.net
</td>
</tr>
<tr>
<th>CC</th>
<td>dgregor@apple.com, llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Compiling this correct program with -std=c++1z:
template<class, class> constexpr bool is_same_v = false;
template<class T> constexpr bool is_same_v<T, T> = true;
template<class...> using void_t = void;
template<class T> struct id { using type = T; };
template<class T> T val() noexcept;
template<class X, class Y> using cond_res =
decltype(false ? val<const X&>() : val<const Y&>());
template<class, class, class = void>
struct CT { using type = void; };
template<class D1, class D2>
#ifndef WORKAROUND
struct CT<D1, D2, void_t<cond_res<D1, D2>>> : id<cond_res<D1, D2>> {};
#else
struct CT<D1, D2, void_t<cond_res<D1, D2>>> {
using type = cond_res<D1, D2>;
};
#endif
struct S1 {};
struct S2 : private S1 {};
struct S3 : protected S1 {};
static_assert(is_same_v<void, CT<S1, S2>::type>); // Fine
static_assert(is_same_v<void, CT<S1, S3>::type>); // Error
produces diagnostices (<a href="https://godbolt.org/z/WGvIFN">https://godbolt.org/z/WGvIFN</a>):
<source>:11:40: error: cannot cast 'const S3' to its protected base class
'const S1'
decltype(false ? val<const X&>() : val<const Y&>());
^
<source>:18:50: note: in instantiation of template type alias 'cond_res'
requested here
struct CT<D1, D2, void_t<cond_res<D1, D2>>> : id<cond_res<D1, D2>> {};
^
<source>:30:31: note: in instantiation of template class 'CT<S1, S3,
void>' requested here
static_assert(is_same_v<void, CT<S1, S3>::type>); // Error
^
<source>:27:13: note: declared protected here
struct S3 : protected S1 {};
^~~~~~~~~~~~
1 error generated.
Private base class? Fine. Protected base class? *EXPLODES*</pre>
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