then fixing the bug will break that code.
New features can also have similar impacts:
enabling the HTTP/2 use by the HTTP client broke programs
connecting to servers with buggy HTTP/2 implementations.
These kinds of changes are unavoidable and
[permitted by the Go 1 compatibility rules](/doc/go1compat).
Even so, Go provides a mechanism called GODEBUG to
reduce the impact such changes have on Go developers
using newer toolchains to compile old code.

    Ordering<Number> unusedK = objects.compound(objects.compound(numbers));

    // You can also arbitrarily assign a more restricted type - not an intended
    // feature, exactly, but unavoidable (I think) and harmless
    Ordering<Integer> unusedL = objects.compound(numbers);

    // This correctly doesn't work:
    // Ordering<Object> unusedM = numbers.compound(objects);

    Ordering<Number> unusedK = objects.compound(objects.compound(numbers));

    // You can also arbitrarily assign a more restricted type - not an intended
    // feature, exactly, but unavoidable (I think) and harmless
    Ordering<Integer> unusedL = objects.compound(numbers);

    // This correctly doesn't work:
    // Ordering<Object> unusedM = numbers.compound(objects);

import java.lang.annotation.RetentionPolicy;
import java.util.Set;

/**
 * Optional features of classes derived from {@code Set}.
 *
 * @author George van den Driessche
 */
@SuppressWarnings("rawtypes") // maybe avoidable if we rework the whole package?
@GwtCompatible
public enum SetFeature implements Feature<Set> {
  GENERAL_PURPOSE(CollectionFeature.GENERAL_PURPOSE);

  private final Set<Feature<? super Set>> implied;

import java.util.List;
import java.util.Set;

/**
 * Optional features of classes derived from {@code List}.
 *
 * @author George van den Driessche
 */
@SuppressWarnings("rawtypes") // maybe avoidable if we rework the whole package?
@GwtCompatible
public enum ListFeature implements Feature<List> {
  SUPPORTS_SET,
  SUPPORTS_ADD_WITH_INDEX(CollectionFeature.SUPPORTS_ADD),

import java.lang.annotation.RetentionPolicy;
import java.util.Set;

/**
 * Optional features of classes derived from {@code Multimap}.
 *
 * @author Louis Wasserman
 */
@SuppressWarnings("rawtypes") // maybe avoidable if we rework the whole package?
@GwtCompatible
public enum MultimapFeature implements Feature<Multimap> {
  VALUE_COLLECTIONS_SUPPORT_ITERATOR_REMOVE;

  private final Set<Feature<? super Multimap>> implied;

import java.lang.annotation.RetentionPolicy;
import java.util.Set;

/**
 * Optional features of classes derived from {@code Set}.
 *
 * @author George van den Driessche
 */
@SuppressWarnings("rawtypes") // maybe avoidable if we rework the whole package?
@GwtCompatible
public enum SetFeature implements Feature<Set> {
  GENERAL_PURPOSE(CollectionFeature.GENERAL_PURPOSE);

  private final Set<Feature<? super Set>> implied;

import java.lang.annotation.RetentionPolicy;
import java.util.Set;

/**
 * Optional features of classes derived from {@link Multiset}.
 *
 * @author Louis Wasserman
 */
@SuppressWarnings("rawtypes") // maybe avoidable if we rework the whole package?
@GwtCompatible
public enum MultisetFeature implements Feature<Multiset> {
  /**
   * Indicates that elements from {@code Multiset.entrySet()} update to reflect changes in the

import java.util.Map;
import java.util.Set;

/**
 * Optional features of classes derived from {@code Map}.
 *
 * @author George van den Driessche
 */
@SuppressWarnings("rawtypes") // maybe avoidable if we rework the whole package?
@GwtCompatible
public enum MapFeature implements Feature<Map> {
  /**
   * The map does not throw {@code NullPointerException} on calls such as {@code containsKey(null)},

import java.util.List;
import java.util.Set;

/**
 * Optional features of classes derived from {@code List}.
 *
 * @author George van den Driessche
 */
@SuppressWarnings("rawtypes") // maybe avoidable if we rework the whole package?
@GwtCompatible
public enum ListFeature implements Feature<List> {
  SUPPORTS_SET,
  SUPPORTS_ADD_WITH_INDEX(CollectionFeature.SUPPORTS_ADD),