Type aliases not being resolved for some functions types

TypeScript Version: 3.4.0-dev.20190310

Search Terms:

Resolve / flatten / simplify type aliases for function calls

resolve type aliases

Code

let subject = {a:1,b:2,c:3,d:4}

type thisResolves = Pick<typeof subject, 'a' | 'b'>

let thisDoesnt = pick(subject, ['a', 'b'])

declare function pick<T, K extends keyof T>(obj: T, keys: K[]): Pick<T,K>

Expected behavior:

The Quick Info type of thisDoesnt resolves to { a: number, b: number}

Actual behavior:

The Quick Info type of thisDoesnt resolves to Pick<{ a: number, b: number, c: number, d: number}, 'a' | 'b'>

Playground Link: here

Related Issues:

I've seen in other issues that type aliases are eagerly resolved (e.g. https://github.com/Microsoft/TypeScript/issues/13095#issuecomment-268627521 and https://github.com/Microsoft/TypeScript/issues/16798#issuecomment-324753135). This is a case where that behavior is useful, since reading through a lot of Pick<Foo<Bar<... in VS Code makes it difficult to figure out the true source of a type error.

Commencez par la reproduction liée dans Playground et examinez la sortie de Quick Info pour thisDoesnt par rapport à thisResolves. Suivez la manière dont le vérificateur de types TypeScript ou le language service présente le résultat de Pick, puis vérifiez que le type affiché est { a: number, b: number } sans modifier le comportement du type sous-jacent.