[Bug c++/66758] [concepts] ICE compiler deeply confused by simple-seeming concept definition

[eric.niebler at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66758

--- Comment #1 from Eric Niebler <eric.niebler at gmail dot com> ---
Created attachment 35907
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=35907&action=edit
preprocessed source, bug #2(?)

This might be the same bug, but the error is different. It yields:
/cygdrive/c/Users/eric/Code/cmcstl2/include/stl2/concepts/foundational.hpp:33:27:
internal compiler error: in tsubst_pack_expansion, at cp/pt.c:10578
     T{ stl2::forward<Args>(args)... };
                           ^