Roll back J2KT `Map.merge` nullness annotations to the more flexible (if possibly less convenient) original version, updating Guava.

This is another annotation from the [On Leveraging Tests to Infer Nullable Annotations](https://github.com/google/guava/issues/6510) data.

RELNOTES=n/a
PiperOrigin-RevId: 546272042

4 files changed, 11 insertions, 5 deletions

diff --git a/guava/src/com/google/common/cache/LocalCache.java b/guava/src/com/google/common/cache/LocalCache.java
index 8206c8c8a..00fb43575 100644
--- a/guava/src/com/google/common/cache/LocalCache.java
+++ b/guava/src/com/google/common/cache/LocalCache.java
@@ -4277,7 +4277,9 @@ class LocalCache<K, V> extends AbstractMap<K, V> implements ConcurrentMap<K, V>
   }
 
   @Override
-  public V merge(K key, V newValue, BiFunction<? super V, ? super V, ? extends V> function) {
+  @CheckForNull
+  public V merge(
+      K key, V newValue, BiFunction<? super V, ? super V, ? extends @Nullable V> function) {
     checkNotNull(key);
     checkNotNull(newValue);
     checkNotNull(function);
diff --git a/guava/src/com/google/common/collect/ImmutableMap.java b/guava/src/com/google/common/collect/ImmutableMap.java
index d1b890a9d..c370e5248 100644
--- a/guava/src/com/google/common/collect/ImmutableMap.java
+++ b/guava/src/com/google/common/collect/ImmutableMap.java
@@ -873,8 +873,9 @@ public abstract class ImmutableMap<K, V> implements Map<K, V>, Serializable {
   @Deprecated
   @Override
   @DoNotCall("Always throws UnsupportedOperationException")
+  @CheckForNull
   public final V merge(
-      K key, V value, BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
+      K key, V value, BiFunction<? super V, ? super V, ? extends @Nullable V> function) {
     throw new UnsupportedOperationException();
   }
 
diff --git a/guava/src/com/google/common/collect/Maps.java b/guava/src/com/google/common/collect/Maps.java
index 3c1e9c03f..173447caf 100644
--- a/guava/src/com/google/common/collect/Maps.java
+++ b/guava/src/com/google/common/collect/Maps.java
@@ -1750,8 +1750,9 @@ public final class Maps {
     }
 
     @Override
+    @CheckForNull
     public V merge(
-        K key, V value, BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
+        K key, V value, BiFunction<? super V, ? super V, ? extends @Nullable V> function) {
       throw new UnsupportedOperationException();
     }
 
@@ -3656,8 +3657,9 @@ public final class Maps {
     }
 
     @Override
+    @CheckForNull
     public V merge(
-        K key, V value, BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
+        K key, V value, BiFunction<? super V, ? super V, ? extends @Nullable V> function) {
       throw new UnsupportedOperationException();
     }
 
diff --git a/guava/src/com/google/common/collect/Synchronized.java b/guava/src/com/google/common/collect/Synchronized.java
index 1274b04cd..b1422e316 100644
--- a/guava/src/com/google/common/collect/Synchronized.java
+++ b/guava/src/com/google/common/collect/Synchronized.java
@@ -1205,8 +1205,9 @@ final class Synchronized {
     }
 
     @Override
+    @CheckForNull
     public V merge(
-        K key, V value, BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
+        K key, V value, BiFunction<? super V, ? super V, ? extends @Nullable V> remappingFunction) {
       synchronized (mutex) {
         return delegate().merge(key, value, remappingFunction);
       }