According to https://github.com/apple/swift/blob/6d2a3bbf9d518c2ff8c637313d1a459f40e5d6bc/docs/TypeChecker.md https://github.com/apple/swift/blob/6d2a3bbf9d518c

According to https://github.com/apple/swift/blob/6d2a3bbf9d518c2ff8c637313d1a459f40e5d6bc/docs/TypeChecker.md https://github.com/apple/swift/blob/6d2a3bbf9d518c2ff8c63731... > The Swift language contains a number of features not part of the Hindley-Milner type system, including constrained polymorphic types and function overloading, which complicate the presentation and implementation somewhat. On the other hand, Swift limits the scope of type inference to a single expression or statement, for purely practical reasons: we expect that we can provide better performance and vastly better diagnostics when the problem is limited in scope. However Hindley-Milner systems are generally linear in complexity whereas Swift's type system experiences combinatorial complexity explosions in the presence of overloads and operators and literals.