[java-dfa] Do not drop NULLABLE when updating dependent variables.

Attempt to fix IDEA-336371 False Negative NPE after instanceof check

GitOrigin-RevId: 1539e33119f58bcd58f20d9015b7c8092d79d02b

java/java-tests/testData/inspection/dataFlow/fixture/NullWarningAfterInstanceofCheck.java

@@ -0,0 +1,21 @@
+import org.jetbrains.annotations.Contract;
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
+
+final class NullFP {
+  void test(@NotNull Y y) {}
+
+  interface X {
+    @Contract(pure = true)
+    @Nullable Y getY();
+  }
+
+  interface Y {}
+  interface Z extends Y {}
+
+  void run(@NotNull final X x) {
+    final Y y = x.getY();
+    if (y instanceof Z) {}
+    test(<warning descr="Argument 'x.getY()' might be null">x.getY()</warning>);
+  }
+}

java/java-tests/testSrc/com/intellij/java/codeInspection/DataFlowInspectionTest.java

@@ -734,4 +734,5 @@ public class DataFlowInspectionTest extends DataFlowInspectionTestCase {
   public void testDuplicatedByPointlessBooleanInspection() { doTest(); }
   public void testSystemOutNullSource() { doTest(); }
   public void testPrimitiveTypeFieldInWrapper() { doTest(); }
+  public void testNullWarningAfterInstanceofCheck() { doTest(); }
 }

platform/analysis-impl/src/com/intellij/codeInspection/dataFlow/memory/DfaMemoryStateImpl.java

@@ -1276,7 +1276,7 @@ public class DfaMemoryStateImpl implements DfaMemoryState {
       type = type.fromRelation(RelationType.EQ);
       for (DfaVariableValue value : eqClass.asList()) {
         if (value != dfaVar) {
-          recordVariableType(value, type);
+          recordVariableType(value, type.meet(value.getInherentType()));
           if (!updateQualifierOnEquality(value, value)) return false;
         }
       }