guava/src/com/google/common/reflect/TypeToken.java
private static WildcardType canonicalizeWildcardType(
      TypeVariable<?> declaration, WildcardType type) {
    Type[] declared = declaration.getBounds();
    List<Type> upperBounds = new ArrayList<>();
    for (Type bound : type.getUpperBounds()) {
      if (!any(declared).isSubtypeOf(bound)) {
        upperBounds.add(canonicalizeWildcardsInType(bound));
      }
    }
    return new Types.WildcardTypeImpl(type.getLowerBounds(), upperBounds.toArray(new Type[0]));
  }
private static WildcardType canonicalizeWildcardType(
      TypeVariable<?> declaration, WildcardType type) {
    Type[] declared = declaration.getBounds();
    List<Type> upperBounds = new ArrayList<>();
    for (Type bound : type.getUpperBounds()) {
      if (!any(declared).isSubtypeOf(bound)) {
        upperBounds.add(canonicalizeWildcardsInType(bound));
      }
    }
    return new Types.WildcardTypeImpl(type.getLowerBounds(), upperBounds.toArray(new Type[0]));
  }
Just don't ever let the user access it.