Add tests for UnusedMethod with overridden methods in private interfaces.

core/src/test/java/com/google/errorprone/bugpatterns/UnusedMethodTest.java

@@ -738,4 +738,92 @@ public int frobnicator() {
             """)
         .doTest();
   }
+
+  @Test
+  public void overriddenMethod_visibleFromSubclass_noFinding() {
+    helper
+        .addSourceLines(
+            "Test.java",
+            """
+            class Test {
+              private interface Frobnicator {
+                int frobnicate();
+              }
+
+              public class Impl implements Frobnicator {
+                @Override
+                public int frobnicate() {
+                  return 0;
+                }
+              }
+            }
+            """)
+        .doTest();
+  }
+
+  @Test
+  public void overriddenMethod_notVisibleOutside() {
+    // Note that while the finding is correct here, the fix is not by itself: the method and all its
+    // overrides could be removed.
+    refactoringHelper
+        .addInputLines(
+            "Test.java",
+            """
+            class Test {
+              private interface Frobnicator {
+                // BUG: Diagnostic contains:
+                int frobnicate();
+              }
+
+              private class Impl implements Frobnicator {
+                @Override
+                public int frobnicate() {
+                  return 0;
+                }
+              }
+            }
+            """)
+        .addOutputLines(
+            "Test.java",
+            """
+            class Test {
+              private interface Frobnicator {}
+
+              private class Impl implements Frobnicator {
+                @Override
+                public int frobnicate() {
+                  return 0;
+                }
+              }
+            }
+            """)
+        .allowBreakingChanges()
+        .doTest();
+  }
+
+  @Test
+  public void overriddenMethod_notVisibleOutside_butUsed() {
+    helper
+        .addSourceLines(
+            "Test.java",
+            """
+            class Test {
+              private interface Frobnicator {
+                int frobnicate();
+              }
+
+              private class Impl implements Frobnicator {
+                @Override
+                public int frobnicate() {
+                  return 0;
+                }
+              }
+
+              void f(Frobnicator f) {
+                var unused = f.frobnicate();
+              }
+            }
+            """)
+        .doTest();
+  }
 }