declare function isFunction<T>(value: T): value is Extract<T, Function>;

declare function toString1(value: object | Function): string ;
declare function toString2(value: Function): string ;

function foo<T>(value: T) {
    if (isFunction(value)) {
        toString1(value); // ok

        toString2(value); // error TS2345: Argument of type 'Extract<T, Function>' is not assignable to parameter of type 'Function'.
                          // Type 'T' is not assignable to type 'Function'.
    }
}