Some information is lost when working with keyof against type arguments

Bug Report

🔎 Search Terms

Hard topic to search for IMO, but I tried phrases like typescript generic interface lose type information and I only found similar issues but not related.

🕗 Version & Regression Information

I was having trouble using version 4.5.5 (that we’re currently using), but I could also reproduce it in the latest version available in the playground (4.6.2). I checked versions back to 3.3.3 in the playground and they all reported the same issue.

⏯ Playground Link

Playground link with relevant code

💻 Code

// BEGIN FRAMEWORK STUFF

type ObjectKey<O, T> = {[K in keyof O]: O[K] extends T ? K : never}[keyof O & string];

interface MyInterface {
    myStringValue: string;
    myIntegerValue: number;
}

abstract class AbstractBase<O> {
    constructor(private readonly object: O) {}

    public getValue<T>(fieldName: ObjectKey<O, T>) {
        return this.object[fieldName];
    }
}

// END FRAMEWORK STUFF

// BEGIN IMPLEMENTATION STUFF

class MyType implements MyInterface {
    myStringValue = "";
    myIntegerValue = -1;
}

class MyInterfaceHolder1<I extends MyInterface> extends AbstractBase<MyInterface> {
    constructor(object: I) {
        super(object);
        this.getValue<number>("myIntegerValue");
    }
}
class MyInterfaceHolder2<I extends MyInterface> extends AbstractBase<I> {
    constructor(object: I) {
        super(object);
        this.getValue<number>("myIntegerValue"); // Argument of type 'string' is not assignable to parameter of type 'ObjectKey<I, number>'.(2345)
    }
}

// END IMPLEMENTATION STUFF

const holder1 = new MyInterfaceHolder1<MyType>(new MyType());
const value1 = holder1.getValue<number>("myIntegerValue");

const holder2 = new MyInterfaceHolder1<MyType>(new MyType());
const value2 = holder2.getValue<number>("myIntegerValue");

🙁 Actual behavior

I lose some type information within the super-class when working with a generic argument.

🙂 Expected behavior

I feel like it should be able to maintain type information here.

[RyanCavanaugh]

Got around to logging #48992

[RyanCavanaugh]

There’s no way for TS to establish the higher-order relationship between what goes into type ObjectKey<O, T> = {[K in keyof O]: O[K] extends T ? K : never}[keyof O & string]; and how that relates to the provided keys. We’d need some built-in operator to be able to deduce that this is safe; being “safe by construction” is not something the checker can recognize.