Type Guards on `this` should affect type parameters

[SLaks]

TypeScript Version: 3.2.1-insiders.20181128

Search Terms: type guard this generic

Summary

In a class MyClass<T>, a type guard of the form this is MyClass<T & Foo> should constrain T to extend Foo.

Alternative

It might make more sense to instead allow type guards on generic parameters; eg, isFoo(): T is Foo.

Code

class FooBuilder<TWith> {
    private bar?: number;
    private qux?: number;

    private whatIsTWith?: TWith

    private hasBar(): this is FooBuilder<TWith | WithBar>{
        return !!this.bar;
    }
    private hasQux(): this is FooBuilder<TWith | WithQux>{
        return !!this.qux;
    }

    public specify(options: Options<TWith>) {
        if (this.hasBar()) {
            options.bar;
            this.whatIsTWith;
        }
        if (this.hasQux()) {
            options.qux;
        }
    }
}

class WithBar { private noStructuralTyping: undefined; }
class WithQux { private noStructuralTyping: undefined; }

type Options<TWith> =
      (WithBar extends TWith ? { bar: string } : {})
    & (WithQux extends TWith ? { qux: string } : {});

// Usage:
new FooBuilder<WithBar | WithQux>().specify({ bar: '', qux: '' });

Expected behavior:

Inside if (this.hasBar()), TWith should be constrained to extend WithBar and options.bar should exist.

Actual behavior:

options still has no properties.

With stricter compilation options, this.whatIsTWith has conflicting definitions.

Playground Link: Link
Related Issues: #14817

[weswigham]

options is unrelated to this - there's no dependency between them. Narrowing this won't narrow options.

[SLaks]

If I move options to a field, this.options still isn't narrowed.

I'm expecting it to affect the TWith type parameter, in all usages.

[typescript-bot]

This issue has been marked as 'Question' and has seen no recent activity. It has been automatically closed for house-keeping purposes. If you're still waiting on a response, questions are usually better suited to stackoverflow.