Make the Opaque type stricter

[resynth1943]

This commit adds a Token parameter to the Opaque types, allowing them to be
fundamentally unique. This means an oaque type cannot be cast to a separate
opaque type. The previous (flawed) design allowed this. This does not.

Closes #70.

[resynth1943]

Should I eslint-ignore that error? Also, I'm seeing linting errors for other tests. Not sure what's going on there.

[papb]

I think you should also add an explicit test that they are really opaque, i.e., add a test that resembles the first post of the issue

[resynth1943]

The thing is, I believe my test resembles the first post of the issue well. It shows opaque types are not interchangeable, which is the desired effect.

[sindresorhus]

Because TypeScript supports recursive types, the Token could be the parent type itself. This could create even stronger unique types. Would be nice with an example in the docs and a test for this. For example:

type Person = {
	id: Opaque<number, Person>;
	name: string;
};

[resynth1943]

@sindresorhus Yes! 😛 I'll add something like that into the TSDoc block.

Thanks.

[resynth1943]

Damn. We gotta get this merged.

[resynth1943]

/bing @sindresorhus @papb

[resynth1943]

Also, I'd like to give apologies for my absence. I've currently been doing my mock examinations in school. Anyway, that's over now, and I'm back. 😛

[sindresorhus]

Thanks :)