TypeScript 条件类型和类型保护

Question

I'm having some problems combining TypeScript's type guards and conditional types. Consider:

export interface IThisThing {
    someProp: number;
}

export function isIThisThing(type: any): type is IThisThing { 
    return !!type.someProp;
}

export interface IThatThing {
    someOtherProp: string;
}

export function isIThatThing(type: any): type is IThatThing { 
    return !!type.someOtherProp;
}

function doAThing<T extends IThisThing | IThatThing>(
    data: T
): T extends IThisThing ? IThisThing : IThatThing {
    if (isIThisThing(data)) { 
        return data; // Type 'T & IThisThing' is not assignable to type 'T extends IThisThing ? IThisThing : IThatThing'.
    };
    return data; // Type 'T' is not assignable to type 'T extends IThisThing ? IThisThing : IThatThing'.
                 //   Type 'IThisThing | IThatThing' is not assignable to type 'T extends IThisThing ? IThisThing : IThatThing'.
                 //     Type 'IThisThing' is not assignable to type 'T extends IThisThing ? IThisThing : IThatThing'.
}

I'd expect the doAThing function to accept IThisThing or IThatThing and to return the same type as it receives. Alas the compiler chokes with messages along the line of:

Type 'T & IThisThing' is not assignable to type 'T extends IThisThing ? IThisThing : IThatThing'.

Can someone set me straight? I feel I'm close but not quite getting it right. I'm using the first example (which seems pretty similar) in this blog post: http://artsy.github.io/blog/2018/11/21/conditional-types-in-typescript/

Answer 1

Typescript will not let you assign anything to a conditional type that still has free type parameter, it's just not supported. Your best bet is to have a signature with the generics and the conditional type and a simpler implementation signature that returns a union of the two possibilities

export interface IThisThing {
    someProp: number;
}

export function isIThisThing(type: any): type is IThisThing { 
    return !!type.someProp;
}

export interface IThatThing {
    someOtherProp: string;
}

export function isIThatThing(type: any): type is IThatThing { 
    return !!type.someOtherProp;
}

function doAThing<T extends IThisThing | IThatThing>(
    data: T
): T extends IThisThing ? IThisThing : IThatThing
function doAThing(
    data: IThisThing | IThatThing
): IThisThing | IThatThing {
    if (isIThisThing(data)) { 
        return data;
    };
  return data;
}

Answer 2

Just return T :

function doAThing<T extends IThisThing | IThatThing>(
    data: T
): T {
    if (isIThisThing(data)) { 
        return data
    } else {
        return data;
    }
}