<html>
<head>
<base href="https://llvm.org/bugs/" />
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<body><table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - is_constructible_v<tuple<move_only>, const move_only&> explodes"
href="https://llvm.org/bugs/show_bug.cgi?id=30979">30979</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>is_constructible_v<tuple<move_only>, const move_only&> explodes
</td>
</tr>
<tr>
<th>Product</th>
<td>libc++
</td>
</tr>
<tr>
<th>Version</th>
<td>unspecified
</td>
</tr>
<tr>
<th>Hardware</th>
<td>All
</td>
</tr>
<tr>
<th>OS</th>
<td>All
</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>All Bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>rs2740@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org, mclow.lists@gmail.com
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>#include <tuple>
struct move_only
{
move_only() = default;
move_only(move_only&&) = default;
};
static_assert(!std::is_constructible_v<std::tuple<move_only>, const move_only
&>);
results in (<a href="http://melpon.org/wandbox/permlink/7eV9ndEz2fYroRgZ">http://melpon.org/wandbox/permlink/7eV9ndEz2fYroRgZ</a>):
In file included from prog.cc:1:
/usr/local/libcxx-head/include/c++/v1/tuple:385:13: error: no matching
constructor for initialization of '__tuple_leaf<0UL, move_only>'
__tuple_leaf<_Uf, _Tf>(_VSTD::forward<_Up>(__u))...,
^ ~~~~~~~~~~~~~~~~~~~~~~~~
/usr/local/libcxx-head/include/c++/v1/tuple:641:11: note: in instantiation of
function template specialization
'std::__1::__tuple_impl<std::__1::__tuple_indices<0>,
move_only>::__tuple_impl<0, move_only, const move_only &>' requested here
: base_(typename __make_tuple_indices<sizeof...(_Tp)>::type(),
^
/usr/local/libcxx-head/include/c++/v1/type_traits:3016:38: note: in
instantiation of function template specialization
'std::__1::tuple<move_only>::tuple<true, false>' requested here
: public integral_constant<bool, __is_constructible(_Tp, _Args...)>
^
/usr/local/libcxx-head/include/c++/v1/type_traits:3200:7: note: in
instantiation of template class
'std::__1::is_constructible<std::__1::tuple<move_only>, const move_only &>'
requested here
= is_constructible<_Tp, _Args...>::value;
^
prog.cc:9:21: note: in instantiation of variable template specialization
'std::__1::is_constructible_v<std::__1::tuple<move_only>, const move_only &>'
requested here
static_assert(!std::is_constructible_v<std::tuple<move_only>, const move_only
&>);
^
/usr/local/libcxx-head/include/c++/v1/tuple:325:5: note: candidate constructor
not viable: no known conversion from 'const move_only' to 'const
std::__1::__tuple_leaf<0, move_only, true>' for 1st argument
__tuple_leaf(__tuple_leaf const &) = default;
^
/usr/local/libcxx-head/include/c++/v1/tuple:326:5: note: candidate constructor
not viable: no known conversion from 'const move_only' to
'std::__1::__tuple_leaf<0, move_only, true>' for 1st argument
__tuple_leaf(__tuple_leaf &&) = default;
^
/usr/local/libcxx-head/include/c++/v1/tuple:300:19: note: candidate template
ignored: disabled by 'enable_if' [with _Tp = const move_only &]
__lazy_and<
^
/usr/local/libcxx-head/include/c++/v1/tuple:286:9: note: candidate constructor
template not viable: requires 2 arguments, but 1 was provided
__tuple_leaf(integral_constant<int, 0>, const _Alloc&) {}
^
/usr/local/libcxx-head/include/c++/v1/tuple:290:9: note: candidate constructor
template not viable: requires 2 arguments, but 1 was provided
__tuple_leaf(integral_constant<int, 1>, const _Alloc& __a)
^
/usr/local/libcxx-head/include/c++/v1/tuple:295:9: note: candidate constructor
template not viable: requires 2 arguments, but 1 was provided
__tuple_leaf(integral_constant<int, 2>, const _Alloc& __a)
^
/usr/local/libcxx-head/include/c++/v1/tuple:312:18: note: candidate constructor
template not viable: requires 3 arguments, but 1 was provided
explicit __tuple_leaf(integral_constant<int, 0>, const _Alloc&, _Tp&&
__t)
^
/usr/local/libcxx-head/include/c++/v1/tuple:317:18: note: candidate constructor
template not viable: requires 3 arguments, but 1 was provided
explicit __tuple_leaf(integral_constant<int, 1>, const _Alloc& __a,
_Tp&& __t)
^
/usr/local/libcxx-head/include/c++/v1/tuple:322:18: note: candidate constructor
template not viable: requires 3 arguments, but 1 was provided
explicit __tuple_leaf(integral_constant<int, 2>, const _Alloc& __a,
_Tp&& __t)
^
/usr/local/libcxx-head/include/c++/v1/tuple:281:49: note: candidate constructor
not viable: requires 0 arguments, but 1 was provided
_LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR __tuple_leaf()
^
prog.cc:9:1: error: static_assert failed
static_assert(!std::is_constructible_v<std::tuple<move_only>, const move_only
&>);
^
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2 errors generated.
I might be reading the code wrong, but it seems to me that for the constructors
taking const _Tp&..., the _CheckArgsConstructor<_Dummy>::template
__enable_implicit<_Tp...>() in the SFINAE condition should use
__enable_implicit<const _Tp&...> instead (and likewise for the explicit
variant).</pre>
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