Do not allow compareTo() to return Int?

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

@@ -1015,10 +1015,8 @@ public class BasicExpressionTypingVisitor extends ExpressionTypingVisitor {
         JetType compareToReturnType = typeInfo.getType();
         JetType type = null;
         if (compareToReturnType != null && !compareToReturnType.isError()) {
-            TypeConstructor constructor = compareToReturnType.getConstructor();
             KotlinBuiltIns builtIns = KotlinBuiltIns.getInstance();
-            TypeConstructor intTypeConstructor = builtIns.getInt().getTypeConstructor();
-            if (constructor.equals(intTypeConstructor)) {
+            if (JetTypeChecker.DEFAULT.equalTypes(builtIns.getIntType(), compareToReturnType)) {
                 type = builtIns.getBooleanType();
             }
             else {

compiler/testData/diagnostics/tests/operatorsOverloading/compareToNullable.kt

@@ -0,0 +1,12 @@
+// !DIAGNOSTICS: -UNUSED_PARAMETER
+
+class C {
+    fun compareTo(c: C): Int? = null
+}
+
+fun test(c: C) {
+    c <!COMPARE_TO_TYPE_MISMATCH!><<!> c
+    c <!COMPARE_TO_TYPE_MISMATCH!><=<!> c
+    c <!COMPARE_TO_TYPE_MISMATCH!>>=<!> c
+    c <!COMPARE_TO_TYPE_MISMATCH!>><!> c
+}

compiler/testData/diagnostics/tests/operatorsOverloading/compareToNullable.txt

@@ -0,0 +1,11 @@
+package
+
+internal fun test(/*0*/ c: C): kotlin.Unit
+
+internal final class C {
+    public constructor C()
+    internal final fun compareTo(/*0*/ c: C): kotlin.Int?
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}

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

@@ -7493,6 +7493,12 @@ public class JetDiagnosticsTestGenerated extends AbstractJetDiagnosticsTest {
                 doTest(fileName);
             }
 
+            @TestMetadata("compareToNullable.kt")
+            public void testCompareToNullable() throws Exception {
+                String fileName = JetTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/operatorsOverloading/compareToNullable.kt");
+                doTest(fileName);
+            }
+
             @TestMetadata("IteratorAmbiguity.kt")
             public void testIteratorAmbiguity() throws Exception {
                 String fileName = JetTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/operatorsOverloading/IteratorAmbiguity.kt");