collect: Return null from Spliterator.getComparator() for natural ordering

Per the Spliterator.getComparator() contract, return null to indicate
natural ordering. This enables stream optimizations where sorted()
becomes a no-op for already-sorted collections using natural ordering.

Previously, ImmutableSortedSet and CollectSpliterators returned
Ordering.natural() instead of null, which caused the JDK stream
implementation to clear the SORTED flag and re-sort unnecessarily.

The fix checks for both Ordering.natural() and Comparator.naturalOrder()
using reference equality (both are singletons).

Fixes #6187

Author: nickita-khylkouski

guava-tests/test/com/google/common/collect/ImmutableSortedSetTest.java

@@ -55,6 +55,7 @@
 import java.util.NoSuchElementException;
 import java.util.Set;
 import java.util.SortedSet;
+import java.util.Spliterator;
 import java.util.TreeSet;
 import java.util.function.BiPredicate;
 import java.util.stream.Collector;
@@ -1225,4 +1226,33 @@ public void testBuilderAsymptotics() {
     assertThat(compares[0]).isAtMost(10000);
     // hopefully something quadratic would have more digits
   }
+
+  @GwtIncompatible // Spliterator
+  public void testSpliteratorComparator_naturalOrdering() {
+    // Per Spliterator.getComparator() contract, null indicates natural ordering
+    ImmutableSortedSet<Integer> set = ImmutableSortedSet.of(1, 2, 3);
+    Spliterator<Integer> spliterator = set.spliterator();
+    assertThat(spliterator.hasCharacteristics(Spliterator.SORTED)).isTrue();
+    assertThat(spliterator.getComparator()).isNull();
+  }
+
+  @GwtIncompatible // Spliterator
+  public void testSpliteratorComparator_customComparator() {
+    // Custom comparator should be returned from getComparator()
+    Comparator<Integer> comparator = Comparator.reverseOrder();
+    ImmutableSortedSet<Integer> set =
+        ImmutableSortedSet.orderedBy(comparator).add(1, 2, 3).build();
+    Spliterator<Integer> spliterator = set.spliterator();
+    assertThat(spliterator.hasCharacteristics(Spliterator.SORTED)).isTrue();
+    assertThat(spliterator.getComparator()).isEqualTo(comparator);
+  }
+
+  @GwtIncompatible // Spliterator
+  public void testAsListSpliteratorComparator_naturalOrdering() {
+    // asList().spliterator() should also return null for natural ordering
+    ImmutableSortedSet<Integer> set = ImmutableSortedSet.of(1, 2, 3);
+    Spliterator<Integer> spliterator = set.asList().spliterator();
+    assertThat(spliterator.hasCharacteristics(Spliterator.SORTED)).isTrue();
+    assertThat(spliterator.getComparator()).isNull();
+  }
 }

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

@@ -53,6 +53,11 @@ private CollectSpliterators() {}
     if (comparator != null) {
       checkArgument((extraCharacteristics & Spliterator.SORTED) != 0);
     }
+    // Per Spliterator.getComparator() contract, return null for natural ordering
+    @Nullable Comparator<? super T> spliteratorComparator =
+        (comparator == Ordering.natural() || comparator == Comparator.naturalOrder())
+            ? null
+            : comparator;
     final class WithCharacteristics implements Spliterator<T> {
       private final Spliterator.OfInt delegate;
 
@@ -92,7 +97,7 @@ public int characteristics() {
       @Override
       public @Nullable Comparator<? super T> getComparator() {
         if (hasCharacteristics(Spliterator.SORTED)) {
-          return comparator;
+          return spliteratorComparator;
         } else {
           throw new IllegalStateException();
         }

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

@@ -833,6 +833,9 @@ public boolean tryAdvance(Consumer<? super E> action) {
 
       @Override
       public Comparator<? super E> getComparator() {
+        if (comparator == Ordering.natural() || comparator == Comparator.naturalOrder()) {
+          return null;
+        }
         return comparator;
       }
     };