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

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

Inizia riproducendo il comportamento di TypeScript 3.7.5 o 3.9.0-dev.20200212 dallo snippet TypeScript dell’issue, confrontando il controllo di uguaglianza diretto con l’equivalente type guard. Traccia la gestione da parte del compilatore del restringimento del tipo per i parametri di tipo generici; il lavoro è completo quando arg === 'a' consente di assegnare arg a T  'a' senza errori.