Type guards are a powerful tool for type system design. They are used to express that a type is only valid if it satisfies a certain condition. For example, we can express that a type is only valid if it is a number or a string. 1. Union Type Guards 1.1. Naive Union Implementation 1.2. 2-adic Union Composition 1.3. N-adic Union Composition 1.3.1. GuardReturnType 1.3.2. Variadic Is-Union 1.3.3. References for this Section 2.