Object literal is not a valid instance of any of the types in the declared type union

[philipahlberg]

TypeScript Version: 3.9.2 (or 3.5.1 and up)

Search Terms:
Union types, object literal

Code

interface AB {
  a: string;
  b: number;
}

interface C {
  c: boolean;
}

const abc: AB | C = {
  b: 123,
  c: true,
};

Expected behavior:
The declaration of the variable abc should not be valid.

The error could be something along the lines of:

The type '{ b: number; c: true }' is not assignable to type 'AB | C'.
	Object literal may only specify known properties, and 'b' does not exist in 'C', and 'c' does not exist in 'AB'.

Actual behavior:
The declaration of the variable abc is valid.

Playground Link: typescriptlang.org

Related Issues: Couldn't find any.

[MartinJohns]

Duplicate of #20863 (and many many more). Excess property checks are not performed for union types.

[RyanCavanaugh]

Notably, object types are not sealed, so an excess property is not a per se error

[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.