[llvm-bugs] [Bug 26357] New: Incorrect deduction of parameter pack

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=26357

            Bug ID: 26357
           Summary: Incorrect deduction of parameter pack
           Product: clang
           Version: trunk
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++11
          Assignee: unassignedclangbugs at nondot.org
          Reporter: ldionne.2 at gmail.com
                CC: dgregor at apple.com, llvm-bugs at lists.llvm.org
    Classification: Unclassified

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: http://melpon.org/wandbox/permlink/5rsGlLeQbKgiLXqM

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