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

Start by running the provided TypeScript sample through the command-line compiler and compare its result with editor behavior. Done means both paths handle the generic call consistently without the reported TS2345 diagnostic.