Add a test for Map.merge()

Author: mernst

checker/tests/nullness/MapMerge.java

@@ -6,7 +6,7 @@ public class MapMerge {
   public static void main(String[] args) {
     Map<String, String> map = new HashMap<>();
     map.put("k", "v");
-    // :: error: (return)
+    // :: error: (dereference.of.nullable)
     map.merge("k", "v", (a, b) -> null).toString();
   }
 

checker/tests/nullness/MapMerge7082.java

@@ -0,0 +1,38 @@
+import java.util.HashMap;
+import java.util.Map;
+import org.checkerframework.checker.nullness.qual.Nullable;
+
+class MyKey {}
+
+class Issue7082a {
+
+  void method(MyKey key) {
+    Map<MyKey, Integer> a = new HashMap<>();
+    a.merge(
+        key,
+        -1,
+        (Integer old, Integer v) -> {
+          if (old + v == 0) {
+            return null;
+          } else {
+            return old + v;
+          }
+        });
+  }
+}
+
+class MapMerge7082 {
+
+  void method(MyKey key) {
+    Map<MyKey, Integer> a = new HashMap<>();
+    a.merge(key, -1, MapMerge7082::mergeFunction);
+  }
+
+  private static @Nullable Integer mergeFunction(Integer old, Integer v) {
+    if (old + v == 0) {
+      return null;
+    } else {
+      return old + v;
+    }
+  }
+}