The whole-program fixpoint collects a self-recursive call's arg types into the
fn's own params. When a recursive call threads a param straight through unchanged
(same arg, same position — e.g. ray-cast passing `hittables` to itself), that arg's
type is the param's own current type: :any until external callers determine it. And
:any is absorbing, so collecting it pinned the param at :any forever — the type a
caller supplied (a vector of records) was lost, and the fn's field reads stayed
generic.
Skip a same-position pass-through arg in the self-recursion collection (contribute
the join identity). It can't add information — param i ⊇ param i is trivial — so
dropping it is sound; the param is still constrained by every external caller and
by any non-pass-through recursive arg. Applies to both self-recursion paths: a
`defn` recursing through its var, and a named fn literal recursing via its
self-local.
This is why ray-cast's `ray` typed (its recursion passes a fresh ray) but
`hittables` didn't (passed through). With the fix, hittables keeps its
vec<Sphere> element type, so hit-all's reduce element — and hit-sphere's reads —
type without any hint: ray tracer 38.4s -> 31.3s (~1.23x) with no annotations.
run-wp.ss: a recursive fn threading a vec param through keeps its element type.
make test / shakesmoke green, selfhost holds, 0 new divergences.
712cab5265