[Bug c++/96592] New: Tuple element w/ member reference to incomplete template type rejected

["cuzdav at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96592

            Bug ID: 96592
           Summary: Tuple element w/ member reference to incomplete
                    template type rejected
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: cuzdav at gmail dot com
  Target Milestone: ---

Created attachment 49049
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=49049&action=edit
source as shown in preview, g++10.2 preprocessed code, and the error messages

This code seems a regression introduced in g++ 10.1, is still in 10.2 and
remains in the trunk.  It works on 9.x and as far back as g++ 6, plus all
versions of clang since 6, icc, msvc, etc.

There are many independent changes that can make it work, including: 
  * making SomeQuery constructor a template constructor (on SessionU)
  * changing the reference to a pointer
  * explicitly declaring Session as an incomplete type and not using
    templates


#include <tuple>

template <typename SessionT>
struct SomeQuery {
    SessionT& session_;
    SomeQuery(SessionT& session) : session_(session) {}
};

template <typename SessionT>
struct Handler {
    std::tuple<SomeQuery<SessionT>> queries_;
    Handler(SessionT& session) : queries_(session) {}
};

struct Session {
    Handler<Session> handler_;
    Session() : handler_{*this} {}
};

int main() {
    Session session;
}


It looks like the tuple class is doing some concept checking that isn't quite
working, but I haven't dug deeply enough to determine if it's a library or
underlying compiler issue.

Live example on Compiler Explorer
https://godbolt.org/z/7naPMx