use decltype(auto) so the return type matches the return type passed in.

Author: brendandahl

system/include/emscripten/val.h

@@ -580,7 +580,7 @@ class EMBIND_VISIBILITY_DEFAULT val {
     using Policy = WithPolicies<FilterTypes<isPolicy, Args...>>;
     auto filteredArgs = Filter<isNotPolicy>(args...);
     return std::apply(
-        [&](auto&&... filteredArgs) {
+        [&](auto&&... filteredArgs) -> decltype(auto) {
           return internalCallWithPolicy<Kind, Policy, Ret>(handle, methodName, std::forward<decltype(filteredArgs)>(filteredArgs)...);
         },
         filteredArgs