`T extends infer I1 extends { K: infer I2 }` infers I2 to unknown about typescript HOT 7 CLOSED

type WithHelper = { [K in API['method'] as `${K}:${(API & { method: K })['path']}`]: API & { method: K } };

type XHRMethod = "GET" | "POST"

// example
type API = { method: "GET", path: "/test" } | { method: "POST", path: "/test" };

/*
type ItWorks = {
    "GET:/test": {
        method: "GET";
        path: "/test";
    };
} | {
    "POST:/test": {
        method: "POST";
        path: "/test";
    };
}
*/
type ItWorks =
  API extends infer I
  ? I extends {
    method: infer IMethod extends XHRMethod;
    path: infer IPath extends string;
  }
  ? { [K in `${IMethod}:${IPath}`]: I }
  : never
  : never
  ;

/*
type ItDoesntWork = {
    [x: `GET:${string}`]: API;
    [x: `POST:${string}`]: API;
}
*/
type ItDoesntWork =
  API extends infer I extends {
    method: infer IMethod extends XHRMethod;
    path: infer IPath extends string;
  }
  ? { [K in `${IMethod}:${IPath}`]: I }
  : never
  ;

type XHRMethod = "GET" | "POST"

// example
type API = { method: "GET", path: "/test" } | { method: "POST", path: "/test" };

/*
type WithoutInferI = {
    "GET:/test": API;
    "POST:/test": API;
}
*/
type WithoutInferI =
    API extends {
        method: infer IMethod extends XHRMethod;
        path: infer IPath extends string;
    }
    ? { [K in `${IMethod}:${IPath}`]: API }
    : never
    ;

/*
type WithInferI = {
    "GET:/test": {
        method: "GET";
        path: "/test";
    };
} | {
    "POST:/test": {
        method: "POST";
        path: "/test";
    };
}
*/
type WithInferI =
    API extends infer I ? I extends {
        method: infer IMethod extends XHRMethod;
        path: infer IPath extends string;
    }
    ? { [K in `${IMethod}:${IPath}`]: I }
    : never : never
    ;