Is Solidity Solid Enough?

We introduce Featherweight Solidity, a calculus formalizing the core features of the Solidity language, thus providing a fundamental step to reason about safety properties of smart contracts’ source code. The formalization includes a static type system that represents the foundation of the Solidity compiler. We show that it prevents some errors whereas many others, such as accesses to a non existing function or state variable, are only detected at runtime and cause interruption and rolling-back of transactions. We then propose a refinement of the type system that is retro-compatible with original Solidity code, and statically captures more errors, such as unsafe casts and unsafe call-back expressions.