generics: bug in type inference when method result used as an argument

The following program doesn't compile, but does compile when a temporary
variable is used to hold the result of the first argument expression.
There should be no error compiling this.

class X<T extends Number> {
    static <T extends Number> X<T> f(X<T> a, X<T> b) {
        return f(f(a, b), b);
    }
}