[Bug c++/83226] New: [7 Regression] std::map with reference T breaks in C++17 mode

[sgunderson at bigfoot dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83226

            Bug ID: 83226
           Summary: [7 Regression] std::map with reference T breaks in
                    C++17 mode
           Product: gcc
           Version: 7.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: sgunderson at bigfoot dot com
  Target Milestone: ---

Hi,

The following code works under GCC for -std=c++14, but breaks under -std=c++17:

#include <map>
#include <utility>

int main(void)
{
  std::map<int, const int &> m;
  std::pair<int, const int &> val(3, 4);
  m.insert(val);  // Compile error.
  m.emplace(3, 4);  // Works.
}

I've looked briefly through the standard, but I can't see anything that
indicates you can't have a const reference as value type (not that I'd
recommend it!). The error messages given are:

In file included from /usr/include/c++/7/bits/stl_iterator.h:66:0,
                 from /usr/include/c++/7/bits/stl_algobase.h:67,
                 from /usr/include/c++/7/bits/stl_tree.h:63,
                 from /usr/include/c++/7/map:60,
                 from test.cc:1:
/usr/include/c++/7/bits/ptr_traits.h: In substitution of ‘template<class _Tp>
template<class _Up> using rebind = _Up* [with _Up = const int&; _Tp =
std::_Rb_tree_node<std::pair<const int, const int&> >]’:
/usr/include/c++/7/bits/ptr_traits.h:147:77:   required by substitution of
‘template<class _Ptr, class _Tp> using __ptr_rebind = typename
std::pointer_traits::rebind<_Tp> [with _Ptr =
std::allocator_traits<std::allocator<std::_Rb_tree_node<std::pair<const int,
const int&> > > >::pointer; _Tp = const int&]’
/usr/include/c++/7/bits/node_handle.h:203:69:   required by substitution of
‘template<class _Key, class _Value, class _NodeAlloc> template<class _Tp> using
__pointer = std::__ptr_rebind<typename
std::allocator_traits<_NodeAlloc>::pointer, _Tp> [with _Tp = std::pair<const
int, const int&>::second_type; _Key = int; _Value = std::pair<const int, const
int&>; _NodeAlloc = std::allocator<std::_Rb_tree_node<std::pair<const int,
const int&> > >]’
/usr/include/c++/7/bits/node_handle.h:206:60:   required from ‘class
std::_Node_handle<int, std::pair<const int, const int&>,
std::allocator<std::_Rb_tree_node<std::pair<const int, const int&> > > >’
test.cc:8:15:   required from here
/usr/include/c++/7/bits/ptr_traits.h:133:28: error: forming pointer to
reference type ‘const int&’
         using rebind = _Up*;
                            ^

Confirmed with 20171109 snapshot. Clang 5.0.0 with the same libstdc++ gives a
similar error, so I believe this is about the standard library, not the
compiler (unless it's an invalid program).

GCC 6.4.0 does not give an error here, so I'm marking this as a regression.