HKTypeLambda

Distributes Lambda inside type bounds. Examples:

type T[X] = U        becomes    type T = [X] -> U
type T[X] <: U       becomes    type T >: Nothing <: ([X] -> U)
type T[X] >: L <: U  becomes    type T >: ([X] -> L) <: ([X] -> U)

The variances of regular TypeBounds types, as well as of match aliases and of opaque aliases are always determined from the given parameters params. The variances of other type aliases are determined from the given parameters only if one of these parameters carries a + or - variance annotation. Type aliases without variance annotation are treated structurally. That is, their parameter variances are determined by how the parameter(s) appear in the result type.

Examples:

type T[X] >: A // X is invariant type T[X] <: List[X] // X is invariant type T[X] = List[X] // X is covariant (determined structurally) opaque type T[X] = List[X] // X is invariant opaque type T[+X] = List[X] // X is covariant type T[A, B] = A => B // A is contravariant, B is covariant (determined structurally) type T[A, +B] = A => B // A is invariant, B is covariant