Fix VerifyError for 'when' of type 'Unit?'

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

@@ -234,9 +234,11 @@ public abstract class StackValue {
             }
         }
         else if (toType.equals(JET_UNIT_TYPE)) {
+            if (!fromType.equals(getType(Void.class))) {
                 pop(v, fromType);
                 putUnitInstance(v);
             }
+        }
         else if (toType.getSort() == Type.ARRAY) {
             v.checkcast(toType);
         }
@@ -431,10 +433,8 @@ public abstract class StackValue {
                 v.aconst(value);
             }
 
-            if (value != null) {
             coerceTo(type, v);
         }
-        }
 
         @Override
         public void condJump(Label label, boolean jumpIfFalse, InstructionAdapter v) {

compiler/testData/codegen/box/unit/nullableUnitInWhen1.kt

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

compiler/testData/codegen/box/unit/nullableUnitInWhen2.kt

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

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

@@ -4812,6 +4812,16 @@ public class BlackBoxCodegenTestGenerated extends AbstractBlackBoxCodegenTest {
             doTest("compiler/testData/codegen/box/unit/nullableUnit.kt");
         }
         
+        @TestMetadata("nullableUnitInWhen1.kt")
+        public void testNullableUnitInWhen1() throws Exception {
+            doTest("compiler/testData/codegen/box/unit/nullableUnitInWhen1.kt");
+        }
+        
+        @TestMetadata("nullableUnitInWhen2.kt")
+        public void testNullableUnitInWhen2() throws Exception {
+            doTest("compiler/testData/codegen/box/unit/nullableUnitInWhen2.kt");
+        }
+        
         @TestMetadata("unitClassObject.kt")
         public void testUnitClassObject() throws Exception {
             doTest("compiler/testData/codegen/box/unit/unitClassObject.kt");