IDEA-73344: Writing unknown value to a nullable variable must reset it's nullability status to unknown, rather than nullable.

java/java-impl/src/com/intellij/codeInspection/dataFlow/DfaMemoryStateImpl.java

@@ -504,7 +504,7 @@ public class DfaMemoryStateImpl implements DfaMemoryState {
     DfaTypeValue dfaType = (DfaTypeValue)dfaCond.getRightOperand();
 
     final DfaVariableState varState = getVariableState(dfaVar);
-    varState.setNullable(varState.isNullable() || dfaType.isNullable());
+    varState.setNullable(dfaType.isNullable());
     return !isNotNull(dfaVar) || varState.setInstanceofValue(dfaType);
   }
 

java/java-tests/testData/inspection/dataFlow/NullableLocalVariable/src/Npe.java

@@ -7,7 +7,7 @@ public class Npe {
      o.hashCode(); // NPE
    }
 
-   Object foo() {
+   @Nullable Object foo() {
      return null;
    }
-}
+}