Wildcards need witness protection

In this paper, we show that the unsoundness discovered by Amin and Tate (2016) in Java’s wildcards is avoidable, even absence of a nullness-aware type system. The key insight paper soundness systems implicitly introduce existential types through subtyping hinges on still making sure there are suitable witness when introducing existentially quantified variables. To approach viable, formalizes core calculus proves it sound. We used static analysis based our to look for potential issues vast corpus Java code found none (with 1 false positive). This confirms both Java's has minimal practical consequence, can avoid entirely with positives.