Tests for common supertype inference

.idea/inspectionProfiles/profiles_settings.xml

@@ -1,6 +1,7 @@
 <component name="InspectionProjectProfileManager">
   <settings>
     <option name="PROJECT_PROFILE" value="idea.default" />
+    <option name="USE_PROJECT_PROFILE" value="true" />
     <version value="1.0" />
   </settings>
 </component>

idea/src/org/jetbrains/jet/lang/types/JetTypeChecker.java

@@ -150,7 +150,7 @@ public class JetTypeChecker {
         return result[0];
     }
 
-    private Type commonSupertype(Collection<Type> types) {
+    public Type commonSupertype(Collection<Type> types) {
         Collection<Type> typeSet = new HashSet<Type>(types);
         assert !typeSet.isEmpty();
         boolean nullable = false;

idea/src/org/jetbrains/jet/lang/types/TypeUtils.java

@@ -59,8 +59,9 @@ public class TypeUtils {
     }
 
     private static boolean canHaveSubtypes(Type type) {
-        // TODO : a nullable type can have a subtype -- a non-nullable version
-
+        if (type.isNullable()) {
+            return true;
+        }
         if (!type.getConstructor().isSealed()) {
             return true;
         }

idea/tests/org/jetbrains/jet/types/JetTypeCheckerTest.java

@@ -87,17 +87,30 @@ public class JetTypeCheckerTest extends LightDaemonAnalyzerTestCase {
         assertType("if (true) null else null", "Nothing?");
 
         assertType("if (true) 1 else '1'", "Any");
+    }
 
-        assertType("if (true) null : Base_T<*>? else null : Derived_T<*>?", "Base_T<*>?");
-        assertType("if (true) null : Base_inT<*>? else null : Derived_T<*>?", "Any?");
-        assertType("if (true) null : DDerived_T<Int>? else null : Derived_T<Int>?", "Derived_T<Int>?");
-        assertType("if (true) null : DDerived_T<Int>? else null : DDerived1_T<Int>?", "Derived_T<Int>?");
+    public void testCommonSupertypes() throws Exception {
+        assertCommonSupertype("Int", "Int", "Int");
 
-        assertType("if (true) null : Base_T<Int>? else null : Base_T<Boolean>?", "Base_T<out Any>?");
-        assertType("if (true) null : Base_T<Int>? else null : Base_T<in Int>?", "Base_T<in Int>?");
-        assertType("if (true) null : Derived_T<Int>? else null : Base_T<in Int>?", "Base_T<in Int>?");
-        assertType("if (true) null : Derived_T<in Int>? else null : Base_T<Int>?", "Base_T<in Int>?");
-        assertType("if (true) null : Base_T<Int>? else null : Base_T<*>?", "Base_T<*>?");
+        assertCommonSupertype("Int", "Int", "Nothing");
+        assertCommonSupertype("Int", "Nothing", "Int");
+        assertCommonSupertype("Nothing", "Nothing", "Nothing");
+
+        assertCommonSupertype("Int?", "Int", "Nothing?");
+        assertCommonSupertype("Nothing?", "Nothing?", "Nothing?");
+
+        assertCommonSupertype("Any", "Int", "Char");
+
+        assertCommonSupertype("Base_T<*>", "Base_T<*>", "Derived_T<*>");
+        assertCommonSupertype("Any", "Base_inT<*>", "Derived_T<*>");
+        assertCommonSupertype("Derived_T<Int>", "DDerived_T<Int>", "Derived_T<Int>");
+        assertCommonSupertype("Derived_T<Int>", "DDerived_T<Int>", "DDerived1_T<Int>");
+
+        assertCommonSupertype("Base_T<out Any>", "Base_T<Int>", "Base_T<Boolean>");
+        assertCommonSupertype("Base_T<in Int>", "Base_T<Int>", "Base_T<in Int>");
+        assertCommonSupertype("Base_T<in Int>", "Derived_T<Int>", "Base_T<in Int>");
+        assertCommonSupertype("Base_T<in Int>", "Derived_T<in Int>", "Base_T<Int>");
+        assertCommonSupertype("Base_T<*>", "Base_T<Int>", "Base_T<*>");
     }
 
     public void testBasicSubtyping() throws Exception {
@@ -246,6 +259,15 @@ public class JetTypeCheckerTest extends LightDaemonAnalyzerTestCase {
         assertSubtypingRelation(type1, type2, false);
     }
 
+    private void assertCommonSupertype(String expected, String... types) {
+        Collection<Type> subtypes = new ArrayList<Type>();
+        for (String type : types) {
+            subtypes.add(makeType(type));
+        }
+        Type result = JetTypeChecker.INSTANCE.commonSupertype(subtypes);
+        assertTrue(result + " != " + expected, JetTypeChecker.INSTANCE.equalTypes(result, makeType(expected)));
+    }
+
     private static void assertSubtypingRelation(String type1, String type2, boolean expected) {
         Type typeNode1 = makeType(type1);
         Type typeNode2 = makeType(type2);