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<base href="https://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - wrong template partial specialization instantiated in constexpr context"
href="https://llvm.org/bugs/show_bug.cgi?id=24770">24770</a>
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<tr>
<th>Summary</th>
<td>wrong template partial specialization instantiated in constexpr context
</td>
</tr>
<tr>
<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>Linux
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>C++11
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>zilla@kayari.org
</td>
</tr>
<tr>
<th>CC</th>
<td>dgregor@apple.com, llvm-bugs@lists.llvm.org
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>This originated at <a href="https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67537">https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67537</a> where
Ville's additional SFINAE constraints for N4387 (improving pair and tuple v3)
cause clang to barf on libstdc++'s tuple.
This is a reduced version, which G++ accepts but Clang rejects:
template<typename T, typename U>
struct pair { };
template<typename... _Elements>
constexpr bool sink(_Elements&&...);
template<bool, typename... _Elements>
struct _TC
{
template<typename... _UElements>
static constexpr bool _MoveConstructibleTuple()
{
return sink( pair<_Elements, _UElements>{} ... );
}
};
template<typename... _Elements>
struct _TC<false, _Elements...>
{
template<typename... _UElements>
static constexpr bool _MoveConstructibleTuple()
{
return false;
}
};
template<bool>
struct enable_if { using type = bool; };
template<>
struct enable_if<false> { };
template<typename... _Elements>
struct tuple
{
template<typename... _UElements> using _TMC =
_TC<(sizeof...(_Elements) == sizeof...(_UElements)), _Elements...>;
template<typename... _UElements, typename
enable_if<_TMC<_UElements...>::template
_MoveConstructibleTuple<_UElements...>()>::type=true>
constexpr tuple(_UElements&&...) { }
constexpr tuple() { }
};
struct X { };
constexpr tuple<X> t{};
Clang says:
c.cc:13:48: error: pack expansion contains parameter packs '_Elements' and
'_UElements' that have different lengths (1 vs. 0)
return sink( pair<_Elements, _UElements>{} ... );
~~~~~~~~~ ~~~~~~~~~~ ^
c.cc:40:52: note: in instantiation of function template specialization
'_TC<true, X>::_MoveConstructibleTuple<>' requested here
enable_if<_TMC<_UElements...>::template
_MoveConstructibleTuple<_UElements...>()>::type=true>
^
c.cc:41:17: note: while substituting prior template arguments into non-type
template parameter [with _UElements = <>]
constexpr tuple(_UElements&&...) { }
^~~~~
c.cc:34:8: note: while substituting deduced template arguments into function
template 'tuple' [with _UElements = <>, $1 = (no value)]
struct tuple
^
1 error generated.
The error comes from a member of _TC<true, X> but the _TMC alias template
should ensure that _TC<false, X> is used when the packs have different sizes.</pre>
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