Type guards not compatible with noImplicitAny

[thodoo]

Hi, thanks for this nice project. We have been using it for some quite complex API responses and found an edge case:

Repro (simplified - the real case has a union of two interfaces):

export interface SomeInterface {
  prop: { key1: string } | { key2: string };
}

Type guard:

export function isSomeInterface(obj: unknown): obj is SomeInterface {
    const typedObj = obj as SomeInterface
    return (
        (typedObj !== null &&
            typeof typedObj === "object" ||
            typeof typedObj === "function") &&
        ((typedObj["prop"] !== null &&
            typeof typedObj["prop"] === "object" ||
            typeof typedObj["prop"] === "function") &&
            typeof typedObj["prop"]["key1"] === "string" ||
            (typedObj["prop"] !== null &&
                typeof typedObj["prop"] === "object" ||
                typeof typedObj["prop"] === "function") &&
            typeof typedObj["prop"]["key2"] === "string")
    )
}

Typescript with strict settings complains about implicit any:

error TS7053: Element implicitly has an 'any' type because expression of type '"key1"' can't be used to index type '{ key1: string; } | { key2: string; }'.
  Property 'key1' does not exist on type '{ key1: string; } | { key2: string; }'.

             typeof typedObj["prop"]["key1"] === "string" ||
                       ~~~~~~~~~~~~~~~~~~~~~~~~

error TS7053: Element implicitly has an 'any' type because expression of type '"key2"' can't be used to index type '{ key1: string; } | { key2: string; }'.
  Property 'key2' does not exist on type '{ key1: string; } | { key2: string; }'.

             typeof typedObj["prop"]["key2"] === "string")

Possible solution: adding "key1" in typedObj["prop"]:

export function isSomeInterface(obj: unknown): obj is SomeInterface {
    const typedObj = obj as SomeInterface
    return (
        (typedObj !== null &&
            typeof typedObj === "object" ||
            typeof typedObj === "function") &&
        ((typedObj["prop"] !== null &&
            typeof typedObj["prop"] === "object" ||
            typeof typedObj["prop"] === "function") &&
            "key1" in typedObj["prop"] &&
            typeof typedObj["prop"]["key1"] === "string" ||
            (typedObj["prop"] !== null &&
                typeof typedObj["prop"] === "object" ||
                typeof typedObj["prop"] === "function") &&
            "key2" in typedObj["prop"] &&
            typeof typedObj["prop"]["key2"] === "string")
    )
}

[rhys-vdw]

Thank you, this is in deed an edge case. Your solution looks good. PR welcome!