Union types unexpected behavior

[rommeras]

TypeScript Version: 4.0.5

Search Terms:
array of union types

Code

type A = {a: number; b?: number;}
type B = {c: number; d: number;}
type C = A | B;

const c: Array<C> = [
  {a: 5, c: 6},
];

Expected behavior:
Type '{ a: number; c: number; }' is not assignable to type 'C'. Property 'd' is missing in type '{ a: number; c: number; }' but required in type 'B'.

Actual behavior:
No errors are shown. Making b non optional property of type A gives expected error

Playground Link: https://www.typescriptlang.org/play?#code/C4TwDgpgBAglC8UDeBDAXFAdgVwLYCMIAnAbinwH4McDiSBfAKFEigCEFkBjavQ0qABNetUkxbQAwpzgAfdiUaMuAe0wBnYFB6wiRFCAA8kgHycA2oyjJ0UAKwAabRgBs9B4wC6JIA
Related Issues:

[MartinJohns]

Duplicate of #20863. Your object literal is compatible with A, and excess property checks are not performed for union types (see mentioned issue).

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.