Scalability and precision by combining expressive type systems and deductive verification

Type systems and modern type checkers can be used very successfully to obtain formal correctness guarantees with little specification overhead. However, type systems in practical scenarios have to trade precision for decidability and scalability. Tools for deductive verification, on the other hand, can prove general properties in more cases than a typical type checker can, but they do not scale well. We present a method to complement the scalability of expressive type systems with the precision of deductive program verification approaches. This is achieved by translating the type uses whose correctness the type checker cannot prove into assertions in a specification language, which can be dealt with by a deductive verification tool. Type uses whose correctness the type checker can prove are instead turned into assumptions to aid the verification tool in finding a proof. Our novel approach is introduced both conceptually for a simple imperative language, and practically by a concrete implementation for the Java programming language. The usefulness and power of our approach has been evaluated by discharging known false positives from a real-world program and by a small case study.