Fix #182.

c++/src/kj/async-inl.h

@@ -703,7 +703,10 @@ struct IdentityFunc<void> {
 };
 template <>
 struct IdentityFunc<Promise<void>> {
-  inline Promise<void> operator()() const { return READY_NOW; }
+  Promise<void> operator()() const;
+  // This can't be inline because it will make the translation unit depend on kj-async. Awkwardly,
+  // Cap'n Proto relies on being able to include this header without creating such a link-time
+  // dependency.
 };
 
 }  // namespace _ (private)

c++/src/kj/async.c++

@@ -941,5 +941,9 @@ void AdapterPromiseNodeBase::onReady(Event& event) noexcept {
   onReadyEvent.init(event);
 }
 
+// -------------------------------------------------------------------
+
+Promise<void> IdentityFunc<Promise<void>>::operator()() const { return READY_NOW; }
+
 }  // namespace _ (private)
 }  // namespace kj