Make UnmodifiableMultiset.removeIf(Predicate) unsupported

Even if there's nothing to remove, `removeIf(Predicate)` should throw an
`UnsupportedOperationException`. This is theoretically an incompatible change, in that some existing code might have been calling `removeIf` with a predicate that is never true. None of Google's internal uses show this problem in tests.

Fixes #6702.
Closes #6703.

RELNOTES=`Multisets.unmodifiableMultiset(set).removeIf(predicate)` now throws an exception always, even if nothing matches `predicate`.
PiperOrigin-RevId: 564750701

guava-gwt/src-super/com/google/common/collect/super/com/google/common/collect/ImmutableCollection.java

@@ -25,6 +25,7 @@
 import java.util.Collections;
 import java.util.Iterator;
 import java.util.Spliterator;
+import java.util.function.Predicate;
 import org.checkerframework.checker.nullness.qual.Nullable;
 
 /**
@@ -64,6 +65,10 @@ public final boolean removeAll(Collection<?> oldElements) {
     throw new UnsupportedOperationException();
   }
 
+  public final boolean removeIf(Predicate<? super E> predicate) {
+    throw new UnsupportedOperationException();
+  }
+
   public final boolean retainAll(Collection<?> elementsToKeep) {
     throw new UnsupportedOperationException();
   }

guava-testlib/src/com/google/common/collect/testing/google/UnmodifiableCollectionTests.java

@@ -224,6 +224,11 @@ public static <E> void assertMultisetIsUnmodifiable(Multiset<E> multiset, E samp
     }
     assertCollectionsAreEquivalent(multiset, copy);
 
+    try {
+      multiset.removeIf(x -> false);
+      fail("removeIf(Predicate) succeeded on unmodifiable collection");
+    } catch (UnsupportedOperationException expected) {
+    }
     assertCollectionsAreEquivalent(multiset, copy);
 
     assertSetIsUnmodifiable(multiset.elementSet(), sampleElement);

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

@@ -195,6 +195,11 @@ public boolean removeAll(Collection<?> elementsToRemove) {
       throw new UnsupportedOperationException();
     }
 
+    @Override
+    public boolean removeIf(java.util.function.Predicate<? super E> filter) {
+      throw new UnsupportedOperationException();
+    }
+
     @Override
     public boolean retainAll(Collection<?> elementsToRetain) {
       throw new UnsupportedOperationException();