Typechecking without Minimum Types: Finite Bases and Backward Checking for $F_{<:}^{\top}$

Abstract

System $F_{<:}^{\top}$ has decidable subtyping but lacks minimum types. We prove that typechecking is decidable for arbitrary explicitly annotated terms with unrestricted subsumption. The difficulty is that type abstraction uses a variable's declared bound to type its body, while quantified subtyping forgets that bound. We construct finite covers that preserve all required subtype comparisons across this change of context. These covers yield a compositional finite typing basis and a decision procedure. We then derive a backward checker that propagates acceptable targets without constructing the complete typing basis. Rectangularity of typing families gives a uniform elimination domain while retaining every result type, even when the canonical type is not least. Finite inverse families transport targets through type substitution. The same inverse specification recovers bound forgetting and yields finite type-avoidance families. Both typechecking procedures are total Rocq functions proved equivalent to the original typing judgment. For a separate symbolic word-RAM realization, we prove a quartic worst-case work bound. We measure family representation sizes and comparison storage in independent OCaml prototypes on synthetic checking judgments.