Currently Merge is limited to only 2 arguments Merge<Destination, Source>. In cases where you want to merge more things, you have to nest it, like so:

type MyComplexType = Merge<
    FinalDestination,
    Merge<
        IntermediateDestination,
        Source,
    >,
>;

This request is just to give some syntactic sugar for that case, like so:

type MyComplexType = Merge<FinalDestination, IntermediateDestination, Source>;