Derive value of generic from parameter

Issue

Is there a way for TypeScript to derive the value of a generic parameter from the type of the parameter that is passed?

Consider the following setup.

type Param<Result> = { bar: Result };

function foo<ResultType, ParamType extends Param<ResultType>>(
  value: ParamType
) {
  //
}

foo({ bar: "foo" });

I want the ResultType to be derived from the value passed to foo, in this case string. Since I do not explicitly set a value for the ResultType param, TypeScript defaults to unknown with

function foo<unknown, {
    bar: string;
}>(value: {
    bar: string;
}): void

while I want

function foo<string, {
    bar: string;
}>(value: {
    bar: string;
}): void

The goal is to type the return value of foo based on Result. I do not think that the approach works at all, but maybe there is another way to achieve the same result?

I am grateful for every hint!

Solution

If your only goal is to get the return type of foo based on ParamType, you could instead say that ParamType is Param<unknown>, then using infer in the return type to get what is "inside":

type Param<Result> = { bar: Result };

function foo<ParamType extends Param<unknown>>(
  value: ParamType
): ParamType extends Param<infer U> ? U : never {
//                         ^^^^^^^ make TypeScript try to infer what's inside

Playground

Answered By - caTS