handle null does not effect for string

[HerrCai0907]

let a = 1;

function foo(): string | null {
  return a > 10 ? null : "";
}

export class B {
  test(): void {
    let t: string | null = foo();
    if (t != null) {
      let v: string = t;  // ERROR TS2322: Type '~lib/string/String | null' is not assignable to type '~lib/string/String'.
    }
  }
}

export function test(): void {
  let t: string | null = foo();
  if (t != null) {
    let v: string = t;   // ERROR TS2322: Type '~lib/string/String | null' is not assignable to type '~lib/string/String'.
  }
}

[dcodeIO]

This is expected in 0.19, since != calls an operator overload as a function. Right now the compiler does not attempt to prove what this function does. Using !== fixes the issue for now as it cannot be overloaded. Related to the long standed triple equals discrepancy. Will be tackled by 0.20 by unifying these cases and special-casing comparisons with null.

[github-actions]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed in one week if no further activity occurs. Thank you for your contributions!