Let's make the example even easier. function convertDate<T extends string | undefined>(isoDate?: string): T { return undefined } 'undefined' is assignable to the constraint of type 'T' Means: What you return in the function (undefined), matches the constraints of your generic type parameter T (extends string | undefined). , but 'T' could be instantiated with a different subtype of constraint 'string | undefined'. Means: TypeScript does not consider that as safe. What if you defined your function like this at compile time: // expects string return type according to generics // but you return undefined in function body const res = convertDate<string>("2019-08-16T16:48:33Z") Then according to your signature, you expect the return type to be string. But at runtime that is not the case! This discrepancy that T can be instantiated with a different subtype (here string) than you return in the function (undefined) is expressed with the TypeScript error.

Both of the below are valid as far as T extends (...args: any[]) => any goes logFn((a, b) => a + b) logFn((a, b, c) => c) But if you refer back to the example I gave, the inner definition as: return (a, b) => fn(a, b); So option 2. will throw an error here, which is why typescript is warning you about it.

In the body of the function you have no control over the instantiation by the calling context, so we have to treat things of type T as opaque boxes and say you can't assign to it. A common mistake or misunderstanding was to constraint a type parameter and then assign to its constraint, for example: function f<T extends boolean>(x: T) { x = true; } f<false>(false); This is still incorrect because the constraint only restricts the upper bound, it does not tell you how precise T may really be.

P can't be assigned {}, since the Generic Type P can be a more defined (or restricted) type.

you can have a Type that is more specific than a boolean like this

The value 10 is assignable to the constraint of T, but it is not assignable to this particular instantiation of T. If there was no error I would passing 10 where 3 is expected!