TypeScript Version: 2.4.2

Code

Link to the playground

type One = {
  kind: true;
  foo: string;
} 

type Two = {
  kind: false;
  foo: boolean;
}

type Options = One | Two;

const fn = (options: Options) => { /* ... */ };

const optionsOne = { kind: true, foo: 'bar' };

/**
 * ERROR:
 * Types of property 'kind' are incompatible.
 * Type 'boolean' is not assignable to type 'false'.
 */
fn(optionsOne);

/**
 * No Error.
 */
fn({ kind: true, foo: 'bar' })

Expected behavior: Should be possible to use true | false to create a discriminated union. It seems like true | false and boolean are not interchangeable.

Actual behavior: TypeScript does not widen the above kind to boolean and thus optionsOne can not be used as input for fn. (That's just my wild guess :-x) It's not possible to use true and false for discriminated unions.