Conditional Mapped Types and Generic Type Parameters

[gamli]

TypeScript Version: 2.8.1 and 2.9.0-dev.20180406

Search Terms: Conditional Mapped Types Generic Type Parameter

Code

// defines a type that contains all properties of Super exactly as they are defined in Super
// and all properties of Sub that are not already defined in Super.
type SubAddsNewPropsToSuper<Sub, Super> = { [KeyOfNewPropsInSub in Exclude<keyof Sub, keyof Super>]: Sub[KeyOfNewPropsInSub] } & Super;

class Controller<TModel> {

    constructor(public model: SubAddsNewPropsToSuper<TModel, { Title: string }>) { }

    setFancyTitle() { 

        this.model.Title = "FANCY"; // works
        this.updateModel({ Title: "FANCY" }); // does not work - why?
        const x: Partial<SubAddsNewPropsToSuper<{ Title: "Foo" }, { Title: string }>> = { Title: "FANCY" }; // works
        const y: Partial<SubAddsNewPropsToSuper<TModel, { Title: string }>> = { Title: "FANCY" }; // does not work - why?
    }

    updateModel(changes: Partial<SubAddsNewPropsToSuper<TModel, { Title: string }>>) {
        // merge "changes" into "this.model"
    }
}

Expected behavior:
The type SubAddsNewPropsToSuper<A, B> should define a type that contains all properties of type B exactly as they are defined in B even if type A defines the same property with a different type (e.g. changes the type of a property from string to "OnlyThisString").
The idea behind this is to circumvent the "problem" if Controller was defined as class Controller<TModel extends { Title: string }>{ ... } in which case TModel might actually change the type of property Title.

Actual behavior:
It works as expected if A and B are both non-generic types but it does not work if A is a generic type.

Playground Link: http://www.typescriptlang.org/play/#src=%0A%2F%2F%20defines%20a%20type%20that%20contains%20all%20properties%20of%20Super%20exactly%20as%20they%20are%20defined%20in%20Super%0A%2F%2F%20and%20all%20properties%20of%20Sub%20that%20are%20not%20already%20defined%20in%20Super.%0Atype%20SubAddsNewPropsToSuper%3CSub%2C%20Super%3E%20%3D%20%7B%20%5BKeyOfNewPropsInSub%20in%20Exclude%3Ckeyof%20Sub%2C%20keyof%20Super%3E%5D%3A%20Sub%5BKeyOfNewPropsInSub%5D%20%7D%20%26%20Super%3B%0A%0Aclass%20Controller%3CTModel%3E%20%7B%0A%0A%20%20%20%20constructor(public%20model%3A%20SubAddsNewPropsToSuper%3CTModel%2C%20%7B%20Title%3A%20string%20%7D%3E)%20%7B%20%7D%0A%0A%20%20%20%20setFancyTitle()%20%7B%20%20%20%20%20%20%20%20%0A%20%20%20%20%20%20%20%20this.model.Title%20%3D%20%22FANCY%22%3B%20%2F%2F%20works%0A%20%20%20%20%20%20%20%20this.updateModel(%7B%20Title%3A%20%22FANCY%22%20%7D)%3B%20%2F%2F%20does%20not%20work%20-%20why%3F%0A%20%20%20%20%20%20%20%20const%20x%3A%20Partial%3CSubAddsNewPropsToSuper%3C%7B%20Title%3A%20%22Foo%22%20%7D%2C%20%7B%20Title%3A%20string%20%7D%3E%3E%20%3D%20%7B%20Title%3A%20%22FANCY%22%20%7D%3B%20%2F%2F%20works%0A%20%20%20%20%20%20%20%20const%20y%3A%20Partial%3CSubAddsNewPropsToSuper%3CTModel%2C%20%7B%20Title%3A%20string%20%7D%3E%3E%20%3D%20%7B%20Title%3A%20%22FANCY%22%20%7D%3B%20%2F%2F%20does%20not%20work%20-%20why%3F%0A%20%20%20%20%7D%0A%0A%20%20%20%20updateModel(changes%3A%20Partial%3CSubAddsNewPropsToSuper%3CTModel%2C%20%7B%20Title%3A%20string%20%7D%3E%3E)%20%7B%0A%20%20%20%20%20%20%20%20%2F%2F%20merge%20%22changes%22%20into%20%22this.model%22%0A%20%20%20%20%7D%0A%7D%0A

Related Issues:

[krryan]

I believe this is related to a problem I had where I was attempting to use Omit<Foo, 'bar'> & { bar: AltBar; }, which worked perfectly when Foo was some defined type, but when Foo was a generic parameter that extends { bar: Bar; baz: Baz; }, the baz property was not available, only bar (as an AltBar) was.

And Omit doesn't even use conditions, it's just

type Diff<T extends string, U extends string> = ({ [P in T]: P } & { [P in U]: never } & { [x: string]: never })[T];
type Omit<T, K extends keyof T> = { [P in Diff<keyof T, K>]: T[P] };

In short, it doesn't seem like TypeScript can apply these sorts of modifications to generic/partially known structures?

[gamli]

Since the issue doesn't have a tag, nor is it closed it might be considered just a question?! I asked the question on StackOverflow.

[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.