Generic type aliases

[ahejlsberg]

With this PR type aliases can be generic. For example:

type Lazy<T> = T | (() => T);

var s: Lazy<string>;
s = "eager";
s = () => "lazy";

type Tree<T> = T | { left: Tree<T>, right: Tree<T> };

var tree: Tree<number> = {
    left: {
        left: 0,
        right: {
            left: 1,
            right: 2
        },
    },
    right: 3
};

interface Tuple<A, B> {
    a: A;
    b: B;
}

type Pair<T> = Tuple<T, T>;

interface TaggedPair<T> extends Pair<T> {
    tag: string;
}

var p: TaggedPair<number>;
p.a = 1;
p.b = 2;
p.tag = "test";

Fixes #1616.

[JsonFreeman]

Does it make sense to move the typeParameters and instantiations here for classes and interfaces as well? Seems like it would be nice if they were all in one place.

[ahejlsberg]

I will think about it. It is generally more convenient to have them on the Type object because that's what we're operating on in most cases. Getting back to the SymbolLinks from the type requires us to do getSymbolLinks(type.symbol) which adds a bit of overhead.

[JsonFreeman]

Seems like this error check is being done in 3 places. Can they be consolidated into one check in getTypeFromTypeReference?

[ahejlsberg]

Not easily since we fan out based on symbol kind and only during that processing figure out if the type requires type arguments or not. Best we could do is a helper we call in the 3 places, but it doesn't really seem to be worth it.

[JsonFreeman]

👍