Rework ImmutableMap.copyOf handling of EnumMap to follow what 06b6f4b did for ImmutableSet.

Plus, expand the comment on `ImmutableSet` to acknowledge a second reason for the `unchecked` warning there.

RELNOTES=n/a
PiperOrigin-RevId: 829442016

Author: cpovirk

guava/src/com/google/common/collect/ImmutableMap.java

@@ -692,12 +692,22 @@ public static <K, V> ImmutableMap<K, V> copyOf(Map<? extends K, ? extends V> map
         return kvMap;
       }
     } else if (map instanceof EnumMap) {
-      @SuppressWarnings("unchecked") // safe since map is not writable
-      ImmutableMap<K, V> kvMap =
-          (ImmutableMap<K, V>)
-              copyOfEnumMap(
-                  (EnumMap<?, ? extends V>) map); // hide K (violates bounds) from J2KT, preserve V.
-      return kvMap;
+      EnumMap<?, ? extends V> clone = ((EnumMap<?, ? extends V>) map).clone();
+      for (Entry<?, ?> entry : clone.entrySet()) {
+        checkEntryNotNull(entry.getKey(), entry.getValue());
+      }
+      ImmutableMap<?, ? extends V> untypedResult = ImmutableEnumMap.asImmutable(clone);
+      /*
+       * The result has the same type argument we started with. We just couldn't express EnumMap<K,
+       * ...> or ImmutableEnumMap<K, ...> along the way because our own <K> isn't <K extends
+       * Enum<K>>.
+       *
+       * We are also performing a covariant cast, potentially from ImmutableMap<K, Sub> to
+       * ImmutableMap<K, Super>. That is safe because no one can add elements to the map.
+       */
+      @SuppressWarnings("unchecked")
+      ImmutableMap<K, V> result = (ImmutableMap<K, V>) untypedResult;
+      return result;
     }
     return copyOf(map.entrySet());
   }
@@ -730,16 +740,6 @@ public static <K, V> ImmutableMap<K, V> copyOf(
     }
   }
 
-  private static <K extends Enum<K>, V> ImmutableMap<K, ? extends V> copyOfEnumMap(
-      EnumMap<?, ? extends V> original) {
-    @SuppressWarnings("unchecked") // the best we could do to make copyOf(Map) compile
-    EnumMap<K, V> copy = new EnumMap<>((EnumMap<K, ? extends V>) original);
-    for (Entry<K, V> entry : copy.entrySet()) {
-      checkEntryNotNull(entry.getKey(), entry.getValue());
-    }
-    return ImmutableEnumMap.asImmutable(copy);
-  }
-
   static final Entry<?, ?>[] EMPTY_ENTRY_ARRAY = new Entry<?, ?>[0];
 
   abstract static class IteratorBasedImmutableMap<K, V> extends ImmutableMap<K, V> {

guava/src/com/google/common/collect/ImmutableSet.java

@@ -187,6 +187,8 @@ public static <E> ImmutableSet<E> copyOf(Collection<? extends E> elements) {
       /*
        * The result has the same type argument we started with. We just couldn't express EnumSet<E>
        * or ImmutableEnumSet<E> along the way because our own <E> isn't <E extends Enum<E>>.
+       *
+       * We are also performing a safe covariant cast to change <? extends E> to <E>.
        */
       @SuppressWarnings("unchecked")
       ImmutableSet<E> result = (ImmutableSet<E>) untypedResult;