Infer types from generic type parameters of base types

[jdaley]

TypeScript should be able to infer type information using the generic type parameters of an extended/implemented generic class or interface.

Example:

interface Apple { }
interface Banana { }
interface Orange { }

interface Tree<TFruit> { }

class AppleTree implements Tree<Apple> { }
class BananaTree implements Tree<Banana> { }
class OrangeTree implements Tree<Orange> { }

function pickFrom<TFruit>(tree: Tree<TFruit>): TFruit {
    return null;
}

var tree = new AppleTree();
var fruit = pickFrom(tree);

fruit is inferred to be type {} but it should be Apple.

Worth noting, if tree is declared to be Tree<Apple> instead than a type that extends Tree<Apple>, then fruit is correctly inferred to be Apple.

[DanielRosenwasser]

I believe this is a consequence of us failing to find candidates for TFruit in the implementing classes.

Note that if you have a property in Tree that uses TFruit, this should work out. For example:

interface Tree<TFruit> { fruit: TFruit[] }

class AppleTree implements Tree<Apple> { fruit: Apple[] }
class BananaTree implements Tree<Banana> { fruit: Banana[] }
class OrangeTree implements Tree<Orange> { fruit: Orange[] }

It might be worth linking to #360 here.

[jdaley]

@DanielRosenwasser Thanks for pointing that out. Not ideal, but I could add a dummy property as a workaround.

[danquirk]

In general:

• Do not define types with no members.
• Do not define generic types that don't use the type parameter type.

Otherwise you will get results like this and all sorts of other undesirable behavior. The inference works correctly as long as there's actual data to make inferences from. You surely do not want a type hierarchy that works this way:

var a: Apple = "yay";
var tree: AppleTree = 1;

Your types need to actually do something or contain some data to be meaningful.