TS allows wrong assignment

[Ailrun]

TypeScript Version: 2.8.1

Search Terms: Type checking, Intersection type, Interface, Optional properties

Code

// Following does not emit any errors (unexpected)
interface Base0 {
    k0: number;
}

type A = Base0
    & { k1?: { file: File, str: string } };

interface B extends Base0 {
    k1?: number;
}

declare const a: A;
const b: B = a;

// However, following emits an error (as expected)
type C =
    & { k1?: { file: File, str: string } };

interface D {
    k1?: number;
}

declare const c: C;
const d: D = c;

Expected behavior: Emits errors for const b: B = a and const d: D = c

Actual behavior: Emits an error only for const d: D = c

Playground Link: https://www.typescriptlang.org/play/index.html#src=%2F%2F%20Following%20does%20not%20emit%20any%20errors.%0D%0Ainterface%20Base0%20%7B%0D%0A%20%20%20%20k0%3A%20number%3B%0D%0A%7D%0D%0A%0D%0Atype%20A%20%3D%20Base0%0D%0A%20%20%20%20%26%20%7B%20k1%3F%3A%20%7B%20file%3A%20File%2C%20str%3A%20string%20%7D%20%7D%3B%0D%0A%0D%0Ainterface%20B%20extends%20Base0%20%7B%0D%0A%20%20%20%20k1%3F%3A%20number%3B%0D%0A%7D%0D%0A%0D%0Adeclare%20const%20a%3A%20A%3B%0D%0Aconst%20b%3A%20B%20%3D%20a%3B%0D%0A%0D%0A%2F%2F%20However%2C%20following%20emits%20an%20error.%0D%0Atype%20C%20%3D%0D%0A%20%20%20%20%26%20%7B%20k1%3F%3A%20%7B%20file%3A%20File%2C%20str%3A%20string%20%7D%20%7D%3B%0D%0A%0D%0Ainterface%20D%20%7B%0D%0A%20%20%20%20k1%3F%3A%20number%3B%0D%0A%7D%0D%0A%0D%0Adeclare%20const%20c%3A%20C%3B%0D%0Aconst%20d%3A%20D%20%3D%20c%3B%0D%0A

Related Issues:

Simplified repro:

interface I {}
type A = I & { x?: string };
interface B { x?: number; }

declare const a: A;
const b: B = a;

[mike-marcacci]

This is especially easy to hit when the refinement occurs across function boundaries. This is a minimal reproduction in the form of a real-world case:

type A = {
  a?: string
}

type B = {
  b?: number
}

type AB = A & B;

function takesAB(arg: AB) {
  return arg.a && arg.b ? arg.a.repeat(arg.b) : ""
}

function takesB(arg: B) {
  return takesAB(arg)
}

const is_B_but_not_A = {
  a: true,
  b: 10
}

takesB(is_B_but_not_A)

While this issue specifically addresses the case where a property is optional, it is related to the secondary issue described here that effects objects with index signatures.

I might recommend that the title of this issue be refined to be a big more descriptive, perhaps something like "Optional properties can be assigned the wrong type"

[RyanCavanaugh]

The case in the OP is now correctly an error in 3.9

[Ailrun]

Great, thanks!