Use generic defaults for "empty" constructors of Array, Map, Set

[aleclarson]

TypeScript Version: 3.4.0-dev.20190202

Search Terms:

• assignment expression
• RHS infer

Code

class Test {
 foo?: Map<object, string>

 getFoo(obj: object) {
 // "foo" is inferred as "Map<any, any>" instead of "Map<object, string>"
 const foo = this.foo || (this.foo = new Map())
 foo.set(obj, 1) // No error!
 }
}

Expected behavior: Infer foo to be Map<object, string>

Actual behavior: Infers foo to be Map<any, any>

Playground Link: Click here

Related Issues: Maybe #7782

[DanielRosenwasser]

This is working as intended. JS always returns the RHS, which you can observe by making this.foo a getter that returns something unrelated.

The real problem is that new Map() creates an instantiation with any. I think we could remove that overload and give it default type arguments.

 interface MapConstructor {
- new(): Map<any, any>;
- new<K, V>(entries?: ReadonlyArray<[K, V]> | null): Map<K, V>;
+ new<K = any, V = any>(entries?: ReadonlyArray<[K, V]> | null): Map<K, V>;
 // ...
 }

[aleclarson]

It's worth noting that array literals are special-cased to work according to my expectations in the OP.

See here

[andrewbranch]

It’s not possible to make this work without breaking other things, primarily preventing people from writing Map<string> and omitting the second type parameter. See #45648 for more detailed notes.