Type not narrowed by === when equivalent type guard works

[kaya3]

TypeScript Version: 3.7.5, also tested on 3.9.0-dev.20200212

Search Terms: equals, type guard, type parameter, narrowing

Code

type AorB = 'a' | 'b';

function isA(t: AorB): t is 'a' {
  return t === 'a';
}

function test<T extends AorB>(arg: T) {
  if (isA(arg)) {
    let a1: T & 'a' = arg; // ok
  }
  if (arg === 'a') {
    let a2: T & 'a' = arg; // error
  }
}

Expected behavior: The condition arg === 'a' should narrow the type of arg to T & 'a' in the second if block, in the same way as the equivalent type guard function does.

Actual behavior: The type of arg is not narrowed in the second if block.

Playground Link: Playground Link

Related Issues: N/A

[maciejsikora]

Ok so the following question is related to this - https://stackoverflow.com/questions/60204107/typescript-doesnt-infer-correct-type-with-generic-extending-string-union-type/60206029#60206029.

As I assume what is happening here is that the condition is valid, as it has no error-prone runtime effect, but the typeguard is not applied by it. And the reason why the condition doesn't work as typguard is a fact that typeguard can work only forall members. Below equivalent typeguard:

function guard<X extends AorB , Y extends AorB >(x: X, y: Y): x is Y { return x == y } // error

It means that conditions behave as typeguards only if left and right side of the condition overlap.
And here we can have situation when provided type doesn't overlap (X can be 'a', Y can be 'b').
Is this assumption correct? I cannot find any information about that in language specification.