Several headers are missing includes for things they use.
type_traits.is_enum needs to export type_traits.integral_constant so that clients can access its value member without explicitly including __type_traits/integral_constant.h themselves.
Make subrange_fwd a peer submodule to subrange rather than a submodule of it, and have subrange export subrange_fwd. That will make it easier to programmatically generate modules for the private detail headers, and it will accomplish the same effect that __ranges/subrange.h will make subrange_kind visible.