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.

Start with the linked Playground reproduction and inspect the Quick Info output for thisDoesnt versus thisResolves. Trace how the TypeScript type checker or language service presents the Pick result, then verify that the displayed type is { a: number, b: number } without changing the underlying type behavior.