CLI-only error calling generic function

TypeScript Version: 3.2.0-dev.20181002

Code

function getFirst<K extends string, T>(obj: Record<K, ReadonlyArray<T>>, key: K): T {
	return obj[key][0];
}

interface I {
	readonly n: ReadonlyArray<number>;
	readonly s: ReadonlyArray<string>;
}

function f(i: I): number {
   return getFirst(i, "n");
}

Expected behavior:

No error.

Actual behavior:

No error in the editor. Error on the command line:

src/a.ts:11:20 - error TS2345: Argument of type 'I' is not assignable to parameter of type 'Record<"n", ReadonlyArray<string>>'.
  Types of property 'n' are incompatible.
    Type 'ReadonlyArray<number>' is not assignable to type 'ReadonlyArray<string>'.
      Type 'number' is not assignable to type 'string'.

11    return getFirst(i, "n");

Related Issues: #19686

[sledorze]

Whats the ‘take the first type’ motivation? (Looks like a can of worms to me)

[RyanCavanaugh]

Rewritten version that is stable between CLI/server

type FetchArr<T, K extends keyof T> = T extends { [x in K]: ReadonlyArray<infer E> } ? E : void;  

function getFirst<T, K extends keyof T>(obj: T, key: K): FetchArr<T, K> {
	return obj[key][0];
}

interface I {
	readonly n: ReadonlyArray<number>;
	readonly s: ReadonlyArray<string>;
}

function f(i: I): number {
  return getFirst(i, "n");
}