Detect the correct conditional return type in Typescript class methods

I found this similar questions: Using type predicates in class methods in TypeScript

But the answer does not include returning a value. For example, what I would like to do is fairly simple:

const resolver = new Resolver(data);
if (resolver.hasMatch()) {
   // Invalid type "string | undefined" of template literal expression.
   console.log(`This is the match: ${resolver.getMatch()}`);
} else {
   console.log('`getMatch` returns `undefined` (no match)');
}

Basically, if you call hasMatch the value of getMatch is a string, otherwise it's undefined.

I tried a few different things:

1. using assert: public hasMatch(): asserts this is { getBestMatch(): string } { but the following type problem occurs when trying to return a value: Type 'boolean' is not assignable to type 'void'.

2. using the same syntax, without assert (this was one of the comments on the other question): public hasMatch(): this is ResolveAcceptLanguage & { getBestMatch(): string } { but it does not work. It looks like it's adding an overload (in VScode) but I can't see it...

Is there a way to do this without using as string in the example I provided?