[Bug c++/86930] New: g++ incorrectly complains about incomplete type in structured binding

[janniksilvanus at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86930

            Bug ID: 86930
           Summary: g++ incorrectly complains about incomplete type in
                    structured binding
           Product: gcc
           Version: 8.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: janniksilvanus at gmail dot com
  Target Milestone: ---

: g++ --version
g++ (SUSE Linux) 8.1.1 20180719 [gcc-8-branch revision 262874]
Copyright (C) 2018 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

: cat incomplete_type.ii 
template <typename Index>
struct IndexPair {
   Index first;
   Index second;
};

using UsedIndexPair = IndexPair<int>;

template <typename Applier>
void call_applier(
   UsedIndexPair const & indices,
   Applier && applier)
{
   auto const [a, b] = indices;
   applier(a, b);
}

void instantiate()
{
   call_applier(
      {0, 1},
      [&](int, int){});
}

: g++ --std=c++1z incomplete_type.ii -c
incomplete_type.ii: In function ‘void call_applier(const UsedIndexPair&,
Applier&&)’:
incomplete_type.ii:14:15: warning: structured binding refers to incomplete
class type ‘const IndexPair<int>’
    auto const [a, b] = indices;

gcc-7.3.1 even crashes on this input:

/opt/rh/devtoolset-7/root/usr/bin/g++ --std=c++1z incomplete_type.ii -c
incomplete_type.ii: In function ‘void call_applier(const UsedIndexPair&,
Applier&&)’:
incomplete_type.ii:14:24: internal compiler error: Segmentation fault
    auto const [a, b] = indices;
                        ^~~~~~~