KT-4310 Check for definite returns

#KT-4310 Fixed

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

@@ -270,8 +270,14 @@ public class ControlStructureTypingUtils {
 
             @Override
             public Void visitIfExpression(@NotNull JetIfExpression ifExpression, CheckTypeContext c) {
-                checkExpressionType(ifExpression.getThen(), c);
-                checkExpressionType(ifExpression.getElse(), c);
+                JetExpression thenBranch = ifExpression.getThen();
+                JetExpression elseBranch = ifExpression.getElse();
+                if (thenBranch == null || elseBranch == null) {
+                    visitExpression(ifExpression, c);
+                    return null;
+                }
+                checkExpressionType(thenBranch, c);
+                checkExpressionType(elseBranch, c);
                 return null;
             }
 

compiler/testData/diagnostics/tests/controlStructures/kt4310.kt

@@ -0,0 +1,11 @@
+package f
+
+fun test(a: Boolean, b: Boolean): Int {
+    return if(a) {
+        1
+    } else {
+        <!TYPE_MISMATCH!>if (b) {
+            3
+        }<!>
+    }    // no error, but must be
+}

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

@@ -1670,6 +1670,11 @@ public class JetDiagnosticsTestGenerated extends AbstractDiagnosticsTestWithEage
                 doTest("compiler/testData/diagnostics/tests/controlStructures/kt1075.kt");
             }
             
+            @TestMetadata("kt4310.kt")
+            public void testKt4310() throws Exception {
+                doTest("compiler/testData/diagnostics/tests/controlStructures/kt4310.kt");
+            }
+            
             @TestMetadata("kt657.kt")
             public void testKt657() throws Exception {
                 doTest("compiler/testData/diagnostics/tests/controlStructures/kt657.kt");