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

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

@@ -234,8 +234,10 @@ public abstract class StackValue {
             }
         }
         else if (toType.equals(JET_UNIT_TYPE)) {
-            pop(v, fromType);
+            if (!fromType.equals(getType(Void.class))) {
-            putUnitInstance(v);
+                pop(v, fromType);
+                putUnitInstance(v);
+            }
         }
         else if (toType.getSort() == Type.ARRAY) {
             v.checkcast(toType);
@@ -431,9 +433,7 @@ public abstract class StackValue {
                 v.aconst(value);
             }
 
-            if (value != null) {
+            coerceTo(type, v);
-                coerceTo(type, v);
-            }
         }
 
         @Override

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");