The following program generates a spurious error message:

class  Foo<X extends Number&NonExistentClass> { 
   <X> Foo<X> m() { return null;}
   Foo<?> f = m();
}

error:

TestX.java:2: cannot find symbol
class  Foo<X extends Number&NonExistentClass> { 
                            ^
  symbol: class NonExistentClass
TestX.java:4: type parameter ? is not within its bound
   Foo<?> f = m();
       ^
TestX.java:4: type parameters of <X>Foo<X> cannot be determined; no unique maximal instance exists for type variable X with upper bounds <any>,Object
   Foo<?> f = m();
               ^
  where X is a type-variable:
    X extends Object declared in method <X>m()
3 errors


The problematic line is this:

"instance exists for type variable X with upper bounds <any>,Object"

an erroneous type is spilling into the error message.