We propose a testing framework for validating static typing procedures in compilers. Our core component is a program generator suitably crafted for producing programs that are likely to trigger typing compiler bugs. One of our main contributions is that our program generator gives rise to transformation-based compiler testing for finding typing bugs. We present two novel approaches (type erasure mutation and type overwriting mutation) that apply targeted transformations to an input program to reveal type inference and soundness compiler bugs respectively. Both approaches are guided by an intra-procedural type inference analysis used to capture type information flow. We implement our techniques as a tool, which we call Hephaestus. The extensibility of Hephaestus enables us to test the compilers of three popular JVM languages: Java, Kotlin, and Groovy. Within nine months of testing, we have found 156 bugs (137 confirmed and 85 fixed) with diverse manifestations and root causes in all the examined compilers. Most of the discovered bugs lie in the heart of many critical components related to static typing, such as type inference.

Blockchains have the potential to revolutionize markets andservices. However, they currently exhibit high latencies and fail to handletransaction loads comparable to those managed by traditional financialsystems.Layer-twoprotocols, built on top of (layer-one) blockchains,avoid disseminating every transaction to the whole network by exchang-ing authenticated transactionsoff-chain. Instead, they utilize the expen-sive and low-rate blockchain only as a recourse for disputes. The promiseof layer-two protocols is to complete off-chain transactions in sub-secondsrather than minutes or hours while retaining asset security, reducing feesand allowing blockchains to scale.We systematize the evolution of layer-two protocols over the period fromthe inception of cryptocurrencies in 2009 until today, structuring themultifaceted body of research on layer-two transactions. Categorizingthe research into payment and state channels, commit-chains and pro-tocols for refereed delegation, we provide a comparison of the protocolsand their properties. We provide a systematization of the associated syn-chronization and routing protocols along with their privacy and securityaspects. This Systematization of Knowledge (SoK) clears the layer-twofog, highlights the potential of layer-two solutions and identifies theirunsolved challenges, indicating propitious avenues of future work.