Merge #104: Fix $Proxy.wrap mapping for empty structs with no fields

90f8b37 Fix $Proxy.wrap mapping for empty structs with no fields (Ryan Ofsky)

Pull request description:

  Trying to pass an empty struct that is annotated with `$Proxy.wrap`  and has no data fields fails because the `BuildOne` overload for the last field number is incorrectly declared to accept an lvalue builder reference instead of a universal reference, and the `CustomBuildField` overload which calls `BuildOne` passes an `rvalue` builder, so the call fails with error:

  ```c++
  include/mp/proxy-types.h:1066:5: error: no matching function for call to 'BuildOne'
   1066 |     BuildOne<0>(local_type, invoke_context, output.init(), value);

  include/mp/proxy-types.h:1050:6: note: candidate function [with index = 0, LocalType = mp::test::FooEmpty, Value = mp::test::FooEmpty, Output = mp::test::messages::FooEmpty::Builder] not viable: expects an lvalue for 3rd argument
   1050 | void BuildOne(TypeList<LocalType> param,
  ```

  The problem did not happen as long as the wrapped struct had at least one field because `CustomBuildField` would call a different `BuildOne` overload for the first field number which does accept rvalue builders, and that `BuildOne` overload would call the next `BuildOne` overload for the next field number passing the builder as an lvalue.

Top commit has no ACKs.

Tree-SHA512: eac746a523a860d07ac59525fe75c928487c90725c43183cb418dd9301438a2110092cbd9585de614ed8bbb614a712cf5b57563d91965671ec932268bb47da23

Author: ryanofsky

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();