Retain data flow info after elvis operator

#KT-2825 In Progress

compiler/frontend/src/org/jetbrains/jet/lang/types/expressions/BasicExpressionTypingVisitor.java

@@ -1171,10 +1171,12 @@ public class BasicExpressionTypingVisitor extends ExpressionTypingVisitor {
                 result = booleanType;
             }
             else if (operationType == JetTokens.ELVIS) {
-                JetType leftType = facade.getTypeInfo(left, context.replaceScope(context.scope)).getType();
-                JetType rightType = right == null
-                                    ? null
-                                    : facade.getTypeInfo(right, contextWithExpectedType.replaceScope(context.scope)).getType();
+                JetTypeInfo leftTypeInfo = facade.getTypeInfo(left, context);
+                JetType leftType = leftTypeInfo.getType();
+                dataFlowInfo = leftTypeInfo.getDataFlowInfo();
+
+                ExpressionTypingContext newContext = contextWithExpectedType.replaceDataFlowInfo(dataFlowInfo).replaceScope(context.scope);
+                JetType rightType = right == null ? null : facade.getTypeInfo(right, newContext).getType();
                 if (leftType != null) {
                     if (!leftType.isNullable()) {
                         context.trace.report(USELESS_ELVIS.on(left, leftType));

compiler/testData/diagnostics/tests/dataFlowInfoTraversal/BinaryExpressionElvis.kt

@@ -0,0 +1,14 @@
+fun foo(x: Int?): Int = x!!
+
+fun elvis(x: Number?): Int {
+    val result = (x as Int?) ?: foo(x)
+    x : Int?
+    return result
+}
+
+
+fun elvisWithRHSTypeInfo(x: Number?): Any? {
+    val result = x ?: x!!
+    <!TYPE_MISMATCH!>x<!> : Int?
+    return result
+}

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

@@ -1161,6 +1161,11 @@ public class JetDiagnosticsTestGenerated extends AbstractDiagnosticsTestWithEage
                 doTest("compiler/testData/diagnostics/tests/dataFlowInfoTraversal/BinaryExpressionContainsConvention.kt");
             }
             
+            @TestMetadata("BinaryExpressionElvis.kt")
+            public void testBinaryExpressionElvis() throws Exception {
+                doTest("compiler/testData/diagnostics/tests/dataFlowInfoTraversal/BinaryExpressionElvis.kt");
+            }
+            
             @TestMetadata("BinaryExpressionEqualsConvention.kt")
             public void testBinaryExpressionEqualsConvention() throws Exception {
                 doTest("compiler/testData/diagnostics/tests/dataFlowInfoTraversal/BinaryExpressionEqualsConvention.kt");