Fix $Proxy.wrap mapping for empty structs with no fields

include/mp/proxy-types.h

@@ -1049,8 +1049,8 @@ void BuildOne(TypeList<LocalType> param,
 template <size_t index, typename LocalType, typename Value, typename Output>
 void BuildOne(TypeList<LocalType> param,
     InvokeContext& invoke_context,
-    Output& output,
-    Value& value,
+    Output&& output,
+    Value&& value,
     typename std::enable_if<index == ProxyType<LocalType>::fields>::type* enable = nullptr)
 {
 }

test/mp/test/foo.capnp

@@ -24,6 +24,7 @@ interface FooInterface $Proxy.wrap("mp::test::FooImplementation") {
     callbackSaved @9 (context :Proxy.Context, arg: Int32) -> (result :Int32);
     callbackExtended @10 (context :Proxy.Context, callback :ExtendedCallback, arg: Int32) -> (result :Int32);
     passCustom @11 (arg :FooCustom) -> (result :FooCustom);
+    passEmpty @12 (arg :FooEmpty) -> (result :FooEmpty);
 }
 
 interface FooCallback $Proxy.wrap("mp::test::FooCallback") {
@@ -46,6 +47,9 @@ struct FooCustom $Proxy.wrap("mp::test::FooCustom") {
     v2 @1 :Int32;
 }
 
+struct FooEmpty $Proxy.wrap("mp::test::FooEmpty") {
+}
+
 struct Pair(T1, T2) {
     first @0 :T1;
     second @1 :T2;

test/mp/test/foo.h

@@ -27,6 +27,10 @@ struct FooCustom
     int v2;
 };
 
+struct FooEmpty
+{
+};
+
 class FooCallback
 {
 public:
@@ -55,6 +59,7 @@ class FooImplementation
     int callbackSaved(int arg) { return m_callback->call(arg); }
     int callbackExtended(ExtendedCallback& callback, int arg) { return callback.callExtended(arg); }
     FooCustom passCustom(FooCustom foo) { return foo; }
+    FooEmpty passEmpty(FooEmpty foo) { return foo; }
     std::shared_ptr<FooCallback> m_callback;
 };
 

test/mp/test/test.cpp

@@ -106,6 +106,8 @@ KJ_TEST("Call FooInterface methods")
     KJ_EXPECT(custom_in.v1 == custom_out.v1);
     KJ_EXPECT(custom_in.v2 == custom_out.v2);
 
+    foo->passEmpty(FooEmpty{});
+
     disconnect_client();
     thread.join();