Multiparty Asynchronous Session Types: A Mechanised Proof of Subject Reduction (Artifact)

  Session types offer a type-based approach to describing the message
  exchange protocols between participants in communication-based
  systems.  Initially, they were introduced in a binary setting, specifying
  communication patterns between two components. With the advent of
  multiparty session types (MPST), the typing discipline was extended
  to arbitrarily many components. In MPST, communication patterns are
  given in terms of global types, an Alice-Bob notation that gives a
  global view of how components interact. A central theorem of MPST is
  subject reduction: a well-typed system remains well-typed after
  reduction. The literature contains some formulations of MPST with
  proofs of subject reduction that have later been shown to be
  incorrect. In this paper, we show that the subject reduction proof
  of the original formulation of MPST by Honda et al. contains some
  flaws.  Additionally, we provide a restriction to the theory and show that,
  for this fragment, subject reduction does indeed hold. Finally, we
  use subject reduction to show that well-typed processes never go
  wrong. All of our proofs are mechanised using the Coq proof
  assistant.
  
  This artifact accompanies our paper. It contains the Coq mechanisation of the theory described therein. For installation, please follow the instructions on our README at https://github.com/Tirore96/subject_reduction/blob/ECOOP2025/README.md