Error inferring tuple type arguments · Issue #16360 · microsoft/TypeScript (original) (raw)

TypeScript Version: nightly (2.4.0-dev.20170608)

Code

interface IPromise { then(onFulfill: (value: T) => IWhenable): IPromise; }

interface QPromise { then(onFulfill: (value: T) => IWhenable): IPromise; }

type IWhenable = IPromise | T;

declare function all<A, B>(promises: IWhenable<[IWhenable, IWhenable]>): IPromise<[A, B]>;

declare const prom1: QPromise<[QPromise, QPromise]>; const x1: IPromise<[string, number]> = all(prom1);

const prom2: IPromise<[IPromise, IPromise]> = prom1; const x2: IPromise<[string, number]> = all(prom2);

Expected behavior:

Since IPromise and QPromise have identical bodies, I would expect these to both work.

Actual behavior:

x1 fails, x2 succeeds.

src/a.ts(14,7): error TS2322: Type 'IPromise<[QPromise<string>, QPromise<number>]>' is not assignable to type 'IPromise<[string, number]>'.
  Type '[QPromise<string>, QPromise<number>]' is not assignable to type '[string, number]'.
    Type 'QPromise<string>' is not assignable to type 'string'.