TypeScript versions 2.3 and 2.4 experiencing issues with generic overloads

After upgrading from typescript 2.2, I encountered an issue with the following example.

interface ILayoutResult {
    id: string;
    data: any;
}
interface ILayout{
    getResult<T extends ILayoutResult | ILayoutResult[] | void>() :T;
}

class test implements ILayout{
    getResult(){
       return {id:"a", data:"aa"} as ILayoutResult ;
    }
}


I faced an error stating that the types of property 'getResult' are incompatible.
    It mentioned that Type '() => ILayoutResult' is not assignable to type '<T extends void | ILayoutResult | ILayoutResult[]>() => T'.
      Also, it pointed out that Type 'ILayoutResult' is not assignable to type 'T'.
typescript

Answer №1

There seems to be a mistake in the code snippet provided. The definition of ILayout allows for the following:

const x: ILayout = ...;
const arr = x.getResult<ILayoutResult[]>();
const j = arr.pop();

However, if x represents an instance of test, the code will encounter an error as it does not actually return an ILayoutResult[].

The intended implementation may have been this, indicating that getResult can potentially return any of the three specified types:

interface ILayout {
    getResult(): ILayoutResult | ILayoutResult[] | void;
}

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