Interface with methods interferes with string literal type inference

[wolfgang42]

TypeScript Version: 2.7.0-dev.20171026

Minimal example:

interface Observable<T> {peek(): T}
declare function obs<T>(val: T): Observable<T>
const t: Observable<'A'> = obs('A')

Expected behavior:
Compilation without errors.

Actual behavior:

src/test.ts(3,7): error TS2322: Type 'Observable<string>' is not assignable to type 'Observable<"A">'.
  Type 'string' is not assignable to type '"A"'.

This error goes away if the interface has no methods or function signature:

interface Observable<T> {}

Alternately, you can redundantly specify the type of the argument to obs():

const t: Observable<'A'> = obs('A' as 'A')

[ghost]

We've had some issues about string literal types in the past but this seems like a new one:

declare function id<T>(val: T): T
declare function arr<T>(val: T): T[]
const x: "a" = id("a");
const y: "a"[] = arr("a");

x works but y doesn't. Temporarily marking as a bug.

[ahejlsberg]

This is a design limitation and is working as intended. See #10676 for context. In the original example we can't tell that T is used only in output positions within Observable<T>. Therefore we widen from 'a' to string. As you point out, you can force the desired result by writing 'a' as 'a'. It would be nice to do better here, but it isn't clear how we could cheaply and consistently determine that a type parameter is used only in output positions.

[ahejlsberg]

@andy-ms Your example is working as it should. Array elements are mutable locations so widening is to be expected.

[ghost]

This doesn't seem to be affected by mutability?

declare function arr<T>(val: T): ReadonlyArray<T>
const y: ReadonlyArray<"a"> = arr("a"); // Error

Observable also only returns a T, not exposing it as a property.
Edit: Oh, I see:

it isn't clear how we could cheaply and consistently determine that a type parameter is used only in output positions.

[kpdonn]

@wolfgang42 Another potential workaround for your example if you are only expecting strings is:

interface Observable<T extends string> {peek(): T}
declare function obs<T extends string>(val: T): Observable<T>
const t: Observable<'A'> = obs('A') // now works

[wolfgang42]

@kpdonn The Observable in my example was actually from a library, so I can't change its declaration. Hopefully someone else will find that useful, though.

[I've since had to rewrite the code that had this issue for other reasons, this is not currently a problem for me any more.]

[ghost]

Another case:

declare function createArray<T>(): Array<T>;
interface I {
    a: number;
    b: true[];
}

const x: I = { a: 0, b: createArray() };

src/a.ts(7,7): error TS2322: Type '{ a: number; b: boolean[]; }' is not assignable to type 'I'.
  Types of property 'b' are incompatible.
    Type 'boolean[]' is not assignable to type 'true[]'.
      Type 'boolean' is not assignable to type 'true'.

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.