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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 --- - Incorrect deduction of parameter pack"
href="https://llvm.org/bugs/show_bug.cgi?id=26357">26357</a>
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<th>Summary</th>
<td>Incorrect deduction of parameter pack
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>All
</td>
</tr>
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<th>OS</th>
<td>All
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>C++11
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>ldionne.2@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>dgregor@apple.com, llvm-bugs@lists.llvm.org
</td>
</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>The following code fails to compile on Clang trunk:
------------------------------------------------------------------------------
#include <cstddef>
#include <utility>
template <std::size_t I, typename T>
struct elt { };
template <typename Indices, typename ...T>
struct indexer;
template <std::size_t ...I, typename ...T>
struct indexer<std::index_sequence<I...>, T...>
: elt<I, T>...
{ };
template <typename ...T>
struct types { };
template <std::size_t ...I, typename ...T>
types<T...> slice_types(elt<I, T> const& ...);
struct x0; struct x1; struct x2; struct x3;
using Indexer = indexer<std::index_sequence<0, 1, 2, 3>, x0, x1, x2, x3>;
using Slice = decltype(slice_types<1, 2>(Indexer{},Indexer{}));
------------------------------------------------------------------------------
The error is
prog.cc:25:24: error: no matching function for call to 'slice_types'
using Slice = decltype(slice_types<1, 2>(Indexer{},Indexer{}));
^~~~~~~~~~~~~~~~~
prog.cc:20:13: note: candidate template ignored: deduced type 'const elt<1UL,
x3> &' of 1st parameter does not match adjusted type 'Indexer' (aka
'indexer<std::__1::integer_sequence<unsigned long, 0, 1, 2, 3>, x0, x1, x2,
x3>') of argument [with I = <1, 2>, T = <x3, x3>]
types<T...> slice_types(elt<I, T> const& ...);
^
1 error generated.
It seems like the `T...` parameter pack is wrongly deduced to `x3, x3` in the
`slice_types` function. GCC accepts this code and does the right thing.
Live example: <a href="http://melpon.org/wandbox/permlink/5rsGlLeQbKgiLXqM">http://melpon.org/wandbox/permlink/5rsGlLeQbKgiLXqM</a></pre>
</div>
</p>
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