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

Comienza reproduciendo el comportamiento de TypeScript 3.7.5 o 3.9.0-dev.20200212 a partir del fragmento de TypeScript del issue, comparando la comprobación de igualdad directa con el type guard equivalente. Rastrea cómo maneja el compilador el estrechamiento de tipos para parámetros de tipo genéricos; se considera terminado cuando arg === 'a' permite asignar arg a T  'a' sin errores.