argument dynamically optional

[WORMSS]

interface PropMap {
    a: {
        b: string | number;
    };
    c: never | undefined;
}

function func<K extends keyof PropMap, P extends PropMap[K]>(k: K, p: P) {}
function func2<K extends keyof PropMap, P extends PropMap[K]>(k: K, p?: P) {}

/* ✅ */ func('a');            // should error, needs prop.
/* ✅ */ func('a', { b: '' }); // should be fine
/* ✅ */ func('a', { c: '' }); // should error, wrong prop.
/* ❌ */ func('c');            // should be fine, I don't want to pass in props for never|undefined.
/* ✅ */ func('c', undefined); // should be fine
/* ✅ */ func('c', { c: '' }); // should error, should be given no prop 


/* ❌ */ func2('a');            // should error, needs prop.
/* ✅ */ func2('a', { b: '' }); // should be fine
/* ✅ */ func2('a', { c: '' }); // should error, wrong prop.
/* ✅ */ func2('c');            // should be fine, I don't want to pass in props for never|undefined.
/* ✅ */ func2('c', undefined); // should be fine
/* ✅ */ func2('c', { c: '' }); // should error, should be given no prop 

Playground

I hope the above makes sense, but I can't seem to make this act nicely. There is a use case where it breaks.
(I would love to split it into 2 functions, but sadly, it's not my call.)

With func the func('c') breaks because it is saying I NEED to supply a property
so I thought, simple fix.. make p optional.
but as you can see from func2 this makes func('a') not throw an error, when I WANT it to, because it has a P that is not never.

other things I tried was

interface PropMap {
    a: {
        b: string | number;
    };
    c: never;
}

which means I can't pass 'undefined' as the 2nd parameter, which is perfectly fine, I can live with that..

[sindresorhus]

How is this related to type-fest?

[WORMSS]

Is it not a type?

[sindresorhus]

I don't understand your question.