A protocol method whose impls all return the same record type has a monomorphic
return. collect-pm-rets! scans the unit's (register-(inline-)method ..) forms,
infers each impl fn's return type, and joins them per method; call-ret-type then
types a (method recv ..) call as that record, so a field read off the result
bare-indexes — e.g. (:ray (scatter m ..)) reads off a Ray. A disagreeing impl
joins to :any and keeps the generic path.
run-protoret.ss: a method with all-record impls bare-indexes + unboxes the field
read; a mixed-return method (one impl returns a number) stays generic. make test /
shakesmoke green, selfhost holds, 0 new divergences.
Foundation for auto-typing record values that flow through protocol dispatch. Does
not yet move the ray tracer: its scatter returns ScatterResult-or-nil (Metal
absorbs some rays), and the nil widens the join to :any — typing a nullable return
soundly needs flow-sensitive narrowing (a guarded (some? x) proves non-nil), filed
separately.
Co-authored-by: Yogthos <yogthos@gmail.com>
