KT-4009 Unnecessary attempt to do a smart cast causes an error

#KT-4009 Fixed
  do not add smart casted types for vars to constraint system

compiler/frontend/src/org/jetbrains/jet/lang/resolve/calls/CandidateResolver.java

@@ -665,6 +665,8 @@ public class CandidateResolver {
 
         DataFlowValue dataFlowValue = DataFlowValueFactory.createDataFlowValue(
                 JetPsiUtil.unwrapFromBlock(argumentExpression), type, trace.getBindingContext());
+        if (!dataFlowValue.isStableIdentifier()) return type;
+
         Set<JetType> possibleTypes = dataFlowInfoForArgument.getPossibleTypes(dataFlowValue);
         if (possibleTypes.isEmpty()) return type;
 

compiler/testData/diagnostics/tests/smartCasts/inference/kt4009.kt

@@ -0,0 +1,21 @@
+trait PsiElement {
+    fun getText(): String
+    fun getParent(): PsiElement
+}
+
+trait JetExpression : PsiElement
+
+fun foo1(e: PsiElement) {
+    var current: PsiElement? = e
+    var first = true
+    while (current != null) {
+        if (current is JetExpression && first) {
+            println(current!!.getText()) // error: smart cast not possible. But it's not needed in fact!
+        }
+
+        current = current?.getParent()
+    }
+}
+
+//from library
+fun println(any: Any?) = throw Exception("$any")

compiler/tests/org/jetbrains/jet/checkers/JetDiagnosticsTestGenerated.java

@@ -5608,6 +5608,11 @@ public class JetDiagnosticsTestGenerated extends AbstractDiagnosticsTestWithEage
                     doTest("compiler/testData/diagnostics/tests/smartCasts/inference/kt2851.kt");
                 }
                 
+                @TestMetadata("kt4009.kt")
+                public void testKt4009() throws Exception {
+                    doTest("compiler/testData/diagnostics/tests/smartCasts/inference/kt4009.kt");
+                }
+                
             }
             
             public static Test innerSuite() {