Fix codegen for closures returning 'Unit?'

Only do a checkcast when we need to coerce Object to Unit: the code was
hopefully type-checked so that it'll only be necessary when the value is either
Unit.VALUE or null

compiler/backend/src/org/jetbrains/jet/codegen/StackValue.java

@@ -242,7 +242,10 @@ public abstract class StackValue {
             }
         }
         else if (toType.equals(JET_UNIT_TYPE)) {
-            if (!fromType.equals(getType(Void.class))) {
+            if (fromType.equals(getType(Object.class))) {
+                v.checkcast(JET_UNIT_TYPE);
+            }
+            else if (!fromType.equals(getType(Void.class))) {
                 pop(v, fromType);
                 putUnitInstance(v);
             }

compiler/testData/codegen/box/unit/closureReturnsNullableUnit.kt

@@ -0,0 +1,8 @@
+fun isNull(x: Unit?) = x == null
+
+fun box(): String {
+    val closure: () -> Unit? = { null }
+    if (!isNull(closure())) return "Fail 1"
+
+    return "OK"
+}

compiler/testData/codegen/box/unit/nullableUnitInWhen3.kt

@@ -0,0 +1,12 @@
+fun foo() {}
+
+fun box(): String {
+    val x = when ("A") {
+        "B" -> foo()
+        else -> null
+    }
+
+    foo()
+    
+    return if (x == null) "OK" else "Fail"
+}

compiler/tests/org/jetbrains/jet/codegen/generated/BlackBoxCodegenTestGenerated.java

@@ -4868,6 +4868,11 @@ public class BlackBoxCodegenTestGenerated extends AbstractBlackBoxCodegenTest {
             JetTestUtils.assertAllTestsPresentByMetadata(this.getClass(), "org.jetbrains.jet.generators.tests.GenerateTests", new File("compiler/testData/codegen/box/unit"), Pattern.compile("^(.+)\\.kt$"), true);
         }
         
+        @TestMetadata("closureReturnsNullableUnit.kt")
+        public void testClosureReturnsNullableUnit() throws Exception {
+            doTest("compiler/testData/codegen/box/unit/closureReturnsNullableUnit.kt");
+        }
+        
         @TestMetadata("kt3634.kt")
         public void testKt3634() throws Exception {
             doTest("compiler/testData/codegen/box/unit/kt3634.kt");
@@ -4898,6 +4903,11 @@ public class BlackBoxCodegenTestGenerated extends AbstractBlackBoxCodegenTest {
             doTest("compiler/testData/codegen/box/unit/nullableUnitInWhen2.kt");
         }
         
+        @TestMetadata("nullableUnitInWhen3.kt")
+        public void testNullableUnitInWhen3() throws Exception {
+            doTest("compiler/testData/codegen/box/unit/nullableUnitInWhen3.kt");
+        }
+        
         @TestMetadata("unitClassObject.kt")
         public void testUnitClassObject() throws Exception {
             doTest("compiler/testData/codegen/box/unit/unitClassObject.kt");