Error message on type predicates is slightly inaccurate.

[DanielRosenwasser]

declare class Container<T> {
    filter<U extends T>(predicate: (x: T) => x is U): Container<U>;
}

interface Animal {
    a
}

interface Dog extends Animal {
    d
}

declare function isDog(x: Animal): boolean;

new Container<Animal>().filter(isDog);
//                             ~~~~~

Expected: Signature '(x: Animal): boolean' must be a type predicate.
Actual: Signature '(x: Animal): boolean' must have a type predicate.