No excess property check for value with valid discriminated union type

[bela53]

TypeScript Version: 3.7.2, Nightly

Search Terms: discriminated union, discriminant, excess property check, literal type, singleton type

Code

type S1 = { d1: "foo"; }
type S2 = { d1: "bar"; isSuccess: true; }
type S3 = { d1: "baz"; isSuccess: false; errorMessage: string; }

type State = S1 | S2 | S3;

const testState: State = {
  d1: "foo",
  isSuccess: true, // no error, why no excess property check here?
  errorMessage: "Error!" // no error, why no excess property check here?
}

Expected behavior:
After having set d1: "foo", TS should recognize that object literal testState belongs to S1 and emit an error when trying to add excess properties isSuccess and/or errorMessage. There should be no other discriminant candidate than d1, because only d1 exists in all three states.

Actual behavior:
There is no compile error.

Playground Link:
here

Related Issues:
Stackoverflow question

#32657 (maybe)
It kinda seems, isSuccess: boolean is interpreted as a second discriminant candidate. You can also substitute isSuccess: true/isSuccess: false with isSuccess: 1/isSuccess: 0 and neither get an error. An error is only raised, when isSuccess is changed to a non-literal type like string. But isSuccess shouldn't be treated as discriminant in the first place!