[Bug c++/97587] New: [coroutines] promise_type constructor is called with original parameters, not parameter copies

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97587

            Bug ID: 97587
           Summary: [coroutines] promise_type constructor is called with
                    original parameters, not parameter copies
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jehelset at gmail dot com
  Target Milestone: ---

Created attachment 49443
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=49443&action=edit
Program that results in assertion-failure due to wrong arguments to promise.

The promise-type constructor should be called with the parameter copies from
the coroutine-frame. I couldn't quite understand that this is supposed to be
the case from the current draft of the standard, but I got some help from Lewis
Baker who also referred me to this rewording, where in the previous version it
is clear: 

https://github.com/cplusplus/draft/commit/0b4ab68a54e197eba9d894a8f91270bb0dcdb546

The attached code triggers an assertion that show that this is not the case on
gcc. I initially had the problem on gcc-10.2.0, but reproduced it on gcc-trunk
on godbolt. Compile either version with -fcoroutines, or see
https://gcc.godbolt.org/z/j16Wqx.

I speculated that the logic here,
https://github.com/gcc-mirror/gcc/blob/master/gcc/cp/coroutines.cc#L4491-L4494,
was inverted. It seemed to pass the copied version of a by-reference parameter,
but the original version of a by-value parameter. The opposite would not
trigger the assertion. But I am not that familiar.