dfa: handle variable comparisons to unknown not-null values

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

@@ -583,7 +583,14 @@ public class DfaMemoryStateImpl implements DfaMemoryState {
       if (dfaLeft instanceof DfaVariableValue) {
         DfaVariableValue dfaVar = (DfaVariableValue)dfaLeft;
         if (isUnknownState(dfaVar)) return true;
-        
+
+        if (!dfaRelation.isInstanceOf()) {
+          if (((DfaTypeValue)dfaRight).isNotNull() && isNull(dfaVar)) {
+            return isNegated;
+          }
+          return true;
+        }
+
         if (isNegated) {
           DfaVariableState newState = getVariableState(dfaVar).withNotInstanceofValue((DfaTypeValue)dfaRight);
           if (newState != null) {

java/java-analysis-impl/src/com/intellij/codeInspection/dataFlow/value/DfaRelationValue.java

@@ -52,7 +52,6 @@ public class DfaRelationValue extends DfaValue {
     }
 
     public DfaRelationValue createRelation(DfaValue dfaLeft, DfaValue dfaRight, IElementType relation, boolean negated) {
-      if (dfaRight instanceof DfaTypeValue && INSTANCEOF_KEYWORD != relation) return null;
       if (PLUS == relation) return null;
 
       if (dfaLeft instanceof DfaVariableValue || dfaLeft instanceof DfaBoxedValue || dfaLeft instanceof DfaUnboxedValue
@@ -169,6 +168,10 @@ public class DfaRelationValue extends DfaValue {
     return myRelation == EQEQ && myIsNegated || myRelation == GT && !myIsNegated || myRelation == GE && myIsNegated;
   }
 
+  public boolean isInstanceOf() {
+    return myRelation == INSTANCEOF_KEYWORD;
+  }
+
   @NonNls public String toString() {
     return (isNegated() ? "not " : "") + myLeftOperand + " " + myRelation + " " + myRightOperand;
   }

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

@@ -0,0 +1,14 @@
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
+
+import java.util.Collection;
+
+class Bar3 {
+
+  public void foo(@Nullable Object element) {
+    final String elementType = element != null ? element.toString() : null;
+    if (elementType == "") {
+      System.out.println(element.hashCode());
+    }
+  }
+}

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

@@ -0,0 +1,16 @@
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
+
+import java.util.Collection;
+
+class Bar3 {
+
+  public void foo(@Nullable Object element) {
+    final String elementType = element != null ? element.toString() : null;
+    if (elementType == nullableString()) {
+      System.out.println(<warning descr="Method invocation 'element.hashCode()' may produce 'java.lang.NullPointerException'">element.hashCode()</warning>);
+    }
+  }
+
+  @Nullable String nullableString() { return null; }
+}

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

@@ -0,0 +1,16 @@
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
+
+import java.util.Collection;
+
+class Bar3 {
+
+  public void foo(@Nullable Object element) {
+    final String elementType = element != null ? element.toString() : null;
+    if (elementType == someString()) {
+      System.out.println(<warning descr="Method invocation 'element.hashCode()' may produce 'java.lang.NullPointerException'">element.hashCode()</warning>);
+    }
+  }
+
+  String someString() { return <warning descr="'null' is returned by the method which is not declared as @Nullable">null</warning>; }
+}

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

@@ -71,6 +71,9 @@ public class DataFlowInspectionTest extends LightCodeInsightFixtureTestCase {
   public void testGeneratedEquals() throws Throwable { doTest(); }
 
   public void testIDEA84489() throws Throwable { doTest(); }
+  public void testComparingNullToNotNull() { doTest(); }
+  public void testComparingNullableToNullable() { doTest(); }
+  public void testComparingNullableToUnknown() { doTest(); }
   public void testComparingToNotNullShouldNotAffectNullity() throws Throwable { doTest(); }
   public void testComparingToNullableShouldNotAffectNullity() throws Throwable { doTest(); }
   public void testStringTernaryAlwaysTrue() throws Throwable { doTest(); }