isUnion incorrect behavior

[salisbury-espinosa]

Bug description

let's say there is a type

type TestType =
  | { optionalField?: number; test1: number; test2: string }
  | { test1: number; test2: string };

so

type OptionalFieldType = TestType['optionalField'];

=> TS2339: Property optionalField does not exist on type TestType
=> its correct, BUT:

IsUnion<TestType> = false

const test: IsUnion<TestType> = true;

=> TS2322: Type true is not assignable to type false

but TestType - union....