Incorrect narrowing of Union type after discrimination when one type is assignable to the other (even when explicit type annotation is present)

リンクされている TypeScript Playground の再現コードから始め、注釈付きの obj1 のケースと動作する obj2 のケースを比較します。assignable な union メンバーに対する type guard の narrowing 動作を追跡し、そのうえで WithoutFlags に追加のプロパティを要求せずに、期待される両方の動作が成立することを確認します。

🔎 Search Terms

"discriminating assignable union", "narrowing unions", "unions simplifying with type guards", "type narrowing with union types", "type discrimination with assignable types", "union type annotation issue", "type guard narrowing problem"

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about Type Guards and Structural Typing

⏯ Playground Link

💻 Code

type ObjWithoutFlags = {withoutFlags: WithoutFlags};
type ObjWithFlags = {withFlags: WithFlags}

export interface WithoutFlags {
  id: string
  // if you add any additional field here, it suddenly works correctly:
  // extraField: any
}

export interface WithFlags {
  id: string
  flags: Array<string>
}

declare const props: ObjWithoutFlags | ObjWithFlags

// broken case:
// regardless of whether I explicitly define that type of `obj1` is a union of `WithFlags | WithoutFlags`,
// TS assumes that `obj1` is always `WithoutFlags` when used in conjunction with type discrimination like so:
const obj1: WithFlags | WithoutFlags = 'withoutFlags' in props ? props.withoutFlags : props.withFlags;

// obj1 is narrowed to `WithoutFlags`, despite having an explicit type annotation

if ('flags' in obj1) {
  // obj is incorrectly narrowed to `WithoutFlags & Record<"flags", unknown>`
  // error, because event.flags is 'unknown', even though it should be an `Array<string>`
  obj1.flags[0]
}

// working case, almost identical:
// there's no discrimination at play, and it works as intended:
declare const obj2: WithFlags | WithoutFlags;

if ('flags' in obj2) {
  // obj is narrowed to `ObjWithFlags`
  // no error, because event.flags is correctly `Array<string>`
  obj2.flags[0]
}

🙁 Actual behavior

When accessing properties of a discriminated union type, where one resulting type is assignable to the other, TypeScript incorrectly narrows down the resulting type to the intersection of its types, rather than their union. Simply adding an additional property to the type that's assignable to the other makes TypeScript switch behavior, and use a union type instead.

This occurs even when an explicit type annotation is present on the variable. This behavior is observed not just with conditional ternary expressions but with any type guards.
This leads to an erroneous assumption, for instance, that the object type only contains the fields of the simpler type - in the example case, it is always of type WithoutFlags, even if the explicit annotation indicates a union of WithFlags | WithoutFlags.

const obj1: {flags: string[], id: string} | {id: string} = 'withoutFlags' in props ? props.withoutFlags : props.withFlags;

if ('flags' in obj1) {
  // Error: obj1.flags is considered 'unknown' instead of `Array<string>`
  obj1.flags[0]
}

🙂 Expected behavior

1. TypeScript should always create unions of possible types, unless both types are mutually assignable. Currently it's enough if one of the types is assignable to the other.

2. When a variable is annotated with an explicit type, TypeScript should disregard any magic inference behavior that happens in its initializer

Additional information about the issue

We've stumbled upon this issue accidentally in the real world, because code that used to work, suddenly was erroring. A refactor of GraphQL Fragments that unified two distinct property types into one, suddenly made TypeScript behave as if only one of those distinct types was correct, and all the excess properties were missing, even after applying a type guard to test for which one of the types is being used.