False negatives and false positives for "Object is possibly 'null'" warning in typescript@next with "complex" if

[donaldpipowitch]

TypeScript Version: 2.1.0-dev.20160807

Code

class Nested {
  data: string;
}

class Foo {
  bar: Nested | null = null;
  baz: number = 1;

  get data(): null | string {
    if (this.bar === null || this.baz !== 1) {
      return null;
    } else {
      return this.bar.data;
             // ^~~~~ Object is possibly 'null'. (BUT IT CAN'T BE NULL.)
    }
  }
}

Expected behavior:

No error.

Actual behavior:

An error.

Details:

This failure must be new. I think I installed typescript@next last week and had no error. I installed it today and it throws an error now.

If a you remove the second if-branch (this.baz !== 1) it works.

This also throws:

  get data(): null | string {
    if (this.bar === null) {
      return null;
    } else if (this.baz !== 1) {
      return null;
    } else {
      return this.bar.data;
             // ^~~~~ Object is possibly 'null'. (BUT IT CAN'T BE NULL.)
    }
  }

This works:

  get data(): null | string {
    if (this.baz !== 1) {
      return null;
    } else if (this.bar === null) {
      return null;
    } else  {
      return this.bar.data;
    }
  }

(!) On the other hand: This works, but could be wrong so it should throw an error!

  get data(): null | string {
    if (this.baz !== 1 || this.bar === null) {
      return null;
    } else {
      return this.bar.data;
    }
  }

[yortus]

This also works:

  get data(): null | string {
    let x = this.bar; // x is Nested | null
    if (x === null || this.baz !== 1) {
      return null;
    } else {
      return x.data; // OK
    }
  }

[ahejlsberg]

Duplicate of #10169, fix available in #10188.