False positive "Object is possibly 'null'" error

[vladimir-tikhonov]

Note: it appears only in the last nightly version, so it's probably caused by one of the pull requests merged yesterday.

webassembly studio: https://webassembly.studio/?f=ih6p4z4fub

The following code is now producing an invalid error during compilation:

class Container {
  public another: Another|null = null;
}

class Another {
  public prop: i32 = 0;
}

export function doStuff( container: Container ): void {
  if ( container.another !== null && container.another.prop > 1 ) { // TS2531: Object is possibly 'null'
    container.another.prop = 2;
  }
}

This was not a case previously and not a case in the current version of typescript.

[dcodeIO]

This is a side-effect of #1046 where I enabled null checking of property accesses after fixing several flow problems preventing it.

Underlying problem this uncovers is that the compiler doesn't currently attempt to null-check repeated accesses to class fields, which can be tricky since other code can theoretically unset the field as a side-effect, and due to static typing we must be 100% sure about that.

Until there's a solution, caching in a local should work:

export function doStuff( container: Container ): void {
  var another = container.another;
  if ( another !== null && another.prop > 1 ) {
    another.prop = 2;
  }
}

[dcodeIO]

To give an example of the challenges involved, this will compile just fine in TS, yet misses the null:

class Test {
  foo: string | null = null;
  get bar() {
    var foo = this.foo;
    this.foo = null;
    return foo;
  }
  set bar(a) {
    this.foo = a;
  }
}

function test() {
  var t = new Test();
  t.bar = "123";
  if (t.bar) {
    console.log(t.bar.toString()); // no type check error, yet a runtime error
  }
}
test();

[webmaster128]

I have another false positive that is simpler:

  // src/index.ts line 38-39
  const nullable: string | null = "a";
  const nonnullable: string = nullable || "fallback";

fails to compile with

ERROR TS2531: Object is possibly 'null'.

   const nonnullable: string = nullable || "fallback";
                               ~~~~~~~~~~~~~~~~~~~~~~
 in src/index.ts(39,30)

ERROR: Compile error

But the expression nullable || "fallback" can never be null.