[Bug c++/90215] New: ICE with lambda in fold expression over comma and assignment

[vittorio.romeo at outlook dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90215

            Bug ID: 90215
           Summary: ICE with lambda in fold expression over comma and
                    assignment
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vittorio.romeo at outlook dot com
  Target Milestone: ---

The following code

    #include <tuple>

    template <class T>
    struct X
    {
        template <class F>
        void f(F f)
        {
            f(0);
        }
    };

    template <class... Xs>
    void bug(X<Xs>... xs)
    {
        std::tuple<Xs...> t;

        std::apply([&](auto&... ys)
        {   
            (xs.f([&](auto y)
            {
                ys = y;
            }), ...);
        }, t);
    }

    int main()
    {
        bug(X<int>{});
    } 

produces an ICE with g++ trunk (version 9.0.1 20190422):

    <source>:22:16: internal compiler error: in tsubst_copy, at cp/pt.c:15551
     22 |             ys = y;
        |             ~~~^~~

The bug can be reproduced on godbolt.org here:
https://gcc.godbolt.org/z/NNLI5p