Partial<T> types not working as expected, when extending another interface

[jamespacileo]

TypeScript Version: 2.7.0-insiders.20180108

Code

interface Vehicle{
    wheels: number;
}

class Driver<T extends Vehicle>{
    car: Partial<T>;
    constructor(){
        this.car = {wheels: 4} 
        // Type '{ wheels: 4; }' is not assignable to type 'Partial<T>'.
        // (property) Driver<T extends Vehicle>.car: Partial<T>
    }
}

[jcalz]

So, this is one of many duplicates of #13442, and I imagine that this will be closed.

Someone may break out an example like:

interface TractorTrailer extends Vehicle {
  wheels: 18;
  cargo: string;
}

const theBandit = new Driver<TractorTrailer>();
theBandit.car.wheels // 4 at runtime, 18 at compile time... oops

But while we're here, could someone explain why this doesn't work?

function explainPlease<T extends Vehicle>() {
  const err1: Partial<T> = null! as { wheels: T['wheels'] }; // error
  const err2: Partial<T> = { wheels: void 0 }; // error
}

If I inspect Partial<T> I see that it has a property wheels of type T['wheels'] | undefined, and all properties are optional, so I'd expect the above to compile without warning. Am I missing something, or is it just that the compiler doesn't actually try to check whether something is assignable to Partial<T>?

[Pmyl]

I can see that some examples seems completely safe, look at this and please tell me if I'm wrong:

interface Entity {
    id: string;
}

function aFunction<TItem extends Entity>(): void {
    const partialItem: Partial<TItem> = { id: '0' };
    // Type { id: "0"; } is not assignable to type Partial<TItem>
}

function anotherFunction<TItem extends Entity>(item: TItem): void {
    const partialItem: Partial<TItem> = item; // this works correctly
    item.id = "new";
    partialItem.id = "new"; // this works correctly
}

I know that TItem extends Entity (pseudo code)
TItem extends Entity => { id: string; ...: any; }
and with Partial everything becomes optional (pseudo code)
Partial<TItem> => { id?: string; ...?: any; }
so the "id" property should be there and everything else can be omitted (like I did), why the compiler is telling me that it's not assignable?

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.