Typescript: Unnecessary widening of function argument's type

[timocov]

TypeScript Version: 2.6.1 (2.7.0-dev.20171102)

Code

type Nominal<Kind extends string, Type> = Type & {
    [Symbol.species]: Kind;
};

type A = Nominal<'A', string>;

declare const a: Set<A>;
declare const b: Set<A>;

const c1 = Array.from(a).concat(Array.from(b)); // Argument of type 'string[]' is not assignable to parameter of type 'Nominal<"A", string> | ReadonlyArray<Nominal<"A", string>>'.
const c2 = Array.from(a).concat(Array.from<A>(b)); // OK

const aa = Array.from(a);
const bb = Array.from(b);
const cc = aa.concat(bb); // OK

Expected behavior:
No errors.

Actual behavior:
With Array.from(a).concat(Array.from(b)) we have an error Argument of type 'string[]' is not assignable to parameter of type 'Nominal<"A", string> | ReadonlyArray<Nominal<"A", string>>'.

In 2.5.3 it works fine.

[andy-ms]

A smaller repro:

type A = string & { n: number };
declare function from<T, U = T>(iterable: Iterable<T>, mapfn?: (v: T, k: number) => U): U[];
declare function concat<T>(a: ReadonlyArray<T>, b: ReadonlyArray<T>): T[];

declare const iterA: Iterable<A>;
const a: A[] = from(iterA); // OK
const cat: A[] = concat(a, from(iterA)); // Error

It would work if you remove U and mapfn from from, but that shouldn't be necessary.
We've had some similar problems with Array and ReadonlyArray in the past.

[mhegazy]

We need to split the from function into two overloads i would say.

[andy-ms]

@mhegazy Should that really be necessary? We usually recommend to unify signatures like that.

[mhegazy]

Should that really be necessary? We usually recommend to unify signatures like that.

The U here is just like map, you want to allow a function to transform the input to a different type and infer that independently.

[ahejlsberg]

This one is actually caused by a subtle issue introduced by #18363. I have a fix that I will put up shortly. Meanwhile, the change to lib.d.ts is probably fine, but strictly speaking isn't needed.