[Bug c++/91847] New: init-capture pack of references requires ... on wrong side

[barry.revzin at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91847

            Bug ID: 91847
           Summary: init-capture pack of references requires ... on wrong
                    side
           Product: gcc
           Version: 9.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: barry.revzin at gmail dot com
  Target Milestone: ---

gcc implements the letter of the standard and allows this:

template <typename... T>
void foo(T&... ts) {
    [...&us=ts]{};
}

While that is valid per the latest working draft, and is the wording specified
in P0780R2, the direction we really want to go in is to put the ... on the
other side of the &. That is, the declaration of 'us' should look the same as
the declaration of 'ts'. This is CWG 2378.

In other words, the correct syntax should be:

template <typename... T>
void foo(T&... ts) {
    [&...us=ts]{};
}

clang implements the CWG 2378 rule, this issue should hopefully be resolved by
the time C++20 ships.