The comparability relationship is not documented in the spec

[DanielRosenwasser]

The comparability and assignment compatibility rules differ only in that

• a union type is comparable to a target type if any of its constituents, rather than all of its constituents, are comparable to that target type,
• whether a type is considered weak has no bearing on whether two types are comparable,
• whether an object type has excess properties has no bearing on whether two types are comparable,
• an object type with some optional property is comparable to an object type containing a property of the same name if it has a compatible type, regardless of whether that property is optional, and
• when relating any two signatures, each signature is always instantiated using type Any for all type arguments.