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Interface with methods interferes with string literal type inference #19497
Description
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')
Activity
ahejlsberg commentedon Oct 26, 2017
This is a design limitation and is working as intended. See #10676 for context. In the original example we can't tell that
Tis used only in output positions withinObservable<T>. Therefore we widen from'a'tostring. 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 commentedon Oct 26, 2017
Andy (Andrewkraft) (@Andy-MS) Your example is working as it should. Array elements are mutable locations so widening is to be expected.
kpdonn commentedon Nov 1, 2017
Wolfgang Faust (@wolfgang42) Another potential workaround for your example if you are only expecting strings is:
wolfgang42 commentedon Nov 1, 2017
Kevin Donnelly (@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.]
typescript-bot commentedon Nov 21, 2017
Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.