could be instantiated with a different subtype of constraint-CodePudding

This has been asked a bunch of times but I'm not quite getting it. I want to have a generic type that performs some mapping from some source type to a destination type:

type Source = {
    valSource: string
}

type Destination = {
    valDest: string
}

type GenericTransform = <S extends Source, D extends Destination>(source: S) => D

const transformFunction: GenericTransform = (s: Source): Destination => {
    return {valDest: 'value for dest'}
}

This gives me

Type '(s: Source) => Destination' is not assignable to type 'TransformInterface'.
    Type 'Destination' is not assignable to type 'D'.
        'Destination' is assignable to the constraint of type 'D', but 'D' could be instantiated with a different subtype of constraint 'Destination'.(2322)

Why does this give me this error on the definition of transformFunction?

CodePudding user response：

Oh I get it now. If it was allowed then I could call my function with a type like

type AnotherDestination = {
    anotherKey: string
} & Destination

which could then not be definitely returned from the transformFunction.