Can we write a custom syntax for Kotlin with a compiler plugin? Like union type:

type newType = typeA | typeB

?

You can hack your way into it but it’s easier to modify just typechecking for a valid shape for example:

typealias NewType = Union<TypeA, TypeB>
val a: NewType = a // or b

val f : TypeA? = a //ok
val g : TypeB? = a //ok
val h : TypeA? = b //ok
val i : TypeB? = b //ok
val z : TypeB? = other // not ok