mapped intersection loses type information

[kolodny]

TypeScript Version: 3.4.0-dev.201xxxxx

Search Terms:
"mapped types", "overload", "intersection"

Code

type ReturnObjects<T> = {
    [K in keyof T]: T[K] extends ((...args: any[]) => any) ?
    T[K] & Returnable<T[K]> :
    never
}

type Returnable<Fun extends (...args: any[]) => any> = {
    returnValue(val: ReturnType<Fun>): void;
}

interface I {
    f(): string;
    f(x: any): number;
}

let argsObj: ReturnObjects<I>;
argsObj.f.returnValue(1) // OK
argsObj.f.returnValue('1') // ERROR?!```

Expected behavior:

argsObj.f.returnValue('1') should not cause a type error

Actual behavior:

argsObj.f.returnValue('1') causes a type error

Playground Link

Related Issues: Couldn't find anything

Note that if I switch the order of the interface, it will only expect the other overload. I believe that typescript is overriding the previous value and not intersecting them

[jack-williams]

The problem here is that interface I contains an overloaded function that is passed to ReturnType. When inferring the return type only the last overload is chosen, as specified here #21496:

When inferring from a type with multiple call signatures (such as the type of an overloaded function), inferences are made from the last signature (which, presumably, is the most permissive catch-all case). It is not possible to perform overload resolution based on a list of argument types (this would require us to support typeof for arbitrary expressions, as suggested in #6606, or something similar).

This is why the number input is ok but the string input is not, and this explains the behaviour you describe here:

Note that if I switch the order of the interface, it will only expect the other overload.