Method returning index type breaks type interference of union types

TypeScript Version: 3.4.0-dev.201xxxxx

Search Terms: union, index type

Code

class Test<Props extends Object> {
  constructor(public o: Props) {}
  get<K extends keyof Props>(k: K) : Props[K] { return this.o[k]; }
}

interface A {t: "A";};
interface B {t: "B";};
type u = Test<A> | Test<B>;

const tmp = new Test<B>({t: 'B'}) as u;

switch(tmp.get('t')) {
  case 'A': break;
  case 'B': break;
}

Expected behavior:

Code compiles with tmp.get('t') having type "A" | "B".

This works for:

Actual behavior:

Compilation fails with error:

Type '"B"' is not comparable to type '"A"'.

Type of tmp.get('t') is only "A".

Problem is not contained to string literals, but also other types. Enountered this bug using immutable-js Record.get().

Playground Link: playground

Related Issues: None found

Simplified very slightly

declare class Test<T> {
  obj: T;
  get<K extends keyof T>(k: K): T[K];
}

interface A { t: "A" }
interface B { t: "B" }

declare const tmp: Test<A> | Test<B>;

switch(tmp.get('t')) {
  case 'A': break;
  case 'B': break;
}

Not a regression - this has been the behavior since 2.1.

We don't even understand why the tmp.get call was allowed in the first place? Generic signatures should not have unified across union type boundaries... 😕

I have a PR open here, but IIRC a recent change anders made should have also fixed this - I'll check.

This now works the way we want it to - the call to get is flagged as an error.

Ryan Cavanaugh (@RyanCavanaugh)

Ryan Cavanaugh (@RyanCavanaugh) there is some additional code in #31023 that makes the call not-an-error, as the type parameters and argument lists in use are identical, so unifying them isn't so bad.

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Method returning index type breaks type interference of union types #30717

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Method returning index type breaks type interference of union types #30717

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RyanCavanaugh commented on Apr 2, 2019

RyanCavanaugh commented on Apr 2, 2019

weswigham commented on Jul 15, 2019

RyanCavanaugh commented on Oct 29, 2019

weswigham commented on Oct 29, 2019

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