Value not assignable to type 'Partial' when using interface extends with generic types

Question

Observe the following:

class Table<ValuesType extends DefaultTableValues = DefaultTableValues>{
    public values: ValuesType;
    constructor(initialValues:ValuesType) {
        this.values=initialValues;
    }

    public set(newValues:Partial<ValuesType>){
        this.values={
            ...this.values,
            ...newValues
        }
    }
}

class User<MoreValues extends UserValues = UserValues> extends Table<MoreValues>{
    constructor(values:MoreValues) {
        super(values);

        this.set({avatar:'some-string'});
        //ERROR - Argument of type {avatar:'some-string'} is not assignable to type Partial
    }
}
interface DefaultTableValues{
    id:string;
}
interface UserValues extends DefaultTableValues{
    avatar:string;
    username:string;
}

I get the following error with this.set: Argument of type {avatar:'some-string'} is not assignable to type Partial

If I change class User<MoreValues extends UserValues = UserValues> extends Table<MoreValues> to class User extends Table<UserValues> then it works, but I lose the ability to pass specific User types and extend the User class.

Shouldn't MoreValues always have the avatar:string property since it extends UserValues?

I have a feeling this is somehow related to this question but I can't figure out how to apply a fix to my case.

score 1 · Accepted Answer · answered Aug 21 '21 at 23:46

You are correct in that the issue is the same in the question you linked. Per said question's accepted answer:

But the ability to subtype doesn't just apply to adding additional properties -- subtyping can include choosing a more restricted set of the domain of the properties themselves.

For example, assume the following interface:

interface RestrictedValues extends UserValues {
    avatar: "nonexistent"
}

This interface does extend UserValues, and is thus assignable to MoreValues. However, its avatar property can only have the value nonexistent, so the Partial<> which is set()'s first argument should only allow the value nonexistent, not any string as UserValues would imply. Take, for example, the following code:

class User<MoreValues extends UserValues = UserValues> extends Table<MoreValues>{
    constructor(values:MoreValues) {
        super(values);

        const thing1: RestrictedValues = { avatar: "nonexistent", id: "", username: "" };
        const thing2: MoreValues = thing1;
    }
}

In this, thing1 cannot be assigned to thing2, since it is not guaranteed that MoreValues will not instantiated with, say, { avatar: "" }, where the only accepted value is an empty string, to which "nonexistent" is not assignable. Therefore, TypeScript throws the error:

Type 'RestrictedValues' is not assignable to type 'MoreValues'.
  'RestrictedValues' is assignable to the constraint of type 'MoreValues', but 'MoreValues' could be instantiated with a different subtype of constraint 'UserValues'.

A simpler example of this is provided in this answer, using only booleans, which I recommend reading if you want to better understand the logic behind this behaviour.

That error is warning you, that your Generic Type P can't be assigned to {}, since the Generic Type P can be a more defined, or restricted, to a particular type that can conflict with the default value.

That means that the value {} can't satisfy all the possible Types that can be used by the Generic Type P.

Let's create another example with only booleans that should be easier to understand:
interface OnlyBoolIdentityInterface<T> {
  (arg: T): T;
}

function onlyBoolGeneric<T extends boolean>(arg: T = false): T {
  return arg;
}
if you define a Type that is more specific than a boolean for example:
type TrueType = true;
and if you specialised the function OnlyBoolIdentityInterface to only support true values like this:
const onlyTrueIdentity: OnlyBoolIdentityInterface<TrueType> = onlyBoolGeneric;
even if TrueType respects the constraint set by T extends boolean the default value arg: T = false is not a TrueType.

This is the situation is what the error is trying to convey to you.

So how can you fix this type of errors?

Or you remove the default value

Or T needs to extend the specialised type of the default param that on my example is false

Or T can interfere directly with params that receive default params

For more context about this error message see the issue that suggested this error message https://github.com/Microsoft/TypeScript/issues/29049.

Value not assignable to type 'Partial' when using interface extends with generic types

1 Answers1