DFA: instanceof for unknown variable still may perform declared type check

Fixes IDEA-191374 DFA: "condition `x instanceof X` can be replaced with a null check" is not reported

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

@@ -817,7 +817,13 @@ public class DfaMemoryStateImpl implements DfaMemoryState {
       }
       if (dfaLeft instanceof DfaVariableValue) {
         DfaVariableValue dfaVar = (DfaVariableValue)dfaLeft;
-        if (isUnknownState(dfaVar)) return true;
+        if (isUnknownState(dfaVar)) {
+          if (relationType == RelationType.IS_NOT) {
+            TypeConstraint constraint = factValue.get(DfaFactType.TYPE_CONSTRAINT);
+            return constraint == null || constraint.withNotInstanceofValue(dfaVar.getDfaType()) != null;
+          }
+          return true;
+        }
 
         switch (relationType) {
           case IS:

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

@@ -0,0 +1,12 @@
+// IDEA-191374
+class B {}
+class A {
+  private B b = new B();
+  void foo() {
+    String s = b != null ? b.toString() : ""; //comment this line and the error will appear
+
+    if (<warning descr="Condition 'b instanceof B' is redundant and can be replaced with a null check">b instanceof B</warning>) {  // no error reported here
+      System.out.println(b);
+    }
+  }
+}

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

@@ -610,4 +610,5 @@ public class DataFlowInspectionTest extends DataFlowInspectionTestCase {
   public void testMergedInitializerAndConstructor() { doTest(); }
   public void testClassMethodsInlining() { doTest(); }
   public void testObjectLocality() { doTest(); }
+  public void testInstanceOfForUnknownVariable() { doTest(); }
 }