diff --git a/Analyser/Static/Acorn/Target.hpp b/Analyser/Static/Acorn/Target.hpp
index 556468fbd..d99a2dfcb 100644
--- a/Analyser/Static/Acorn/Target.hpp
+++ b/Analyser/Static/Acorn/Target.hpp
@@ -40,6 +40,10 @@ struct ArchimedesTarget: public ::Analyser::Static::Target, public Reflection::S
 	std::string main_program;
 
 	ArchimedesTarget() : Analyser::Static::Target(Machine::Archimedes) {}
+
+private:
+	friend Reflection::StructImpl<ArchimedesTarget>;
+	void declare_fields() {}
 };
 
 }
diff --git a/OSBindings/Mac/Clock Signal.xcodeproj/xcshareddata/xcschemes/Clock Signal.xcscheme b/OSBindings/Mac/Clock Signal.xcodeproj/xcshareddata/xcschemes/Clock Signal.xcscheme
index dbea90fe3..1dec5b8a5 100644
--- a/OSBindings/Mac/Clock Signal.xcodeproj/xcshareddata/xcschemes/Clock Signal.xcscheme	
+++ b/OSBindings/Mac/Clock Signal.xcodeproj/xcshareddata/xcschemes/Clock Signal.xcscheme	
@@ -24,8 +24,8 @@
    </BuildAction>
    <TestAction
       buildConfiguration = "Debug"
-      selectedDebuggerIdentifier = ""
-      selectedLauncherIdentifier = "Xcode.IDEFoundation.Launcher.PosixSpawn"
+      selectedDebuggerIdentifier = "Xcode.DebuggerFoundation.Debugger.LLDB"
+      selectedLauncherIdentifier = "Xcode.DebuggerFoundation.Launcher.LLDB"
       shouldUseLaunchSchemeArgsEnv = "YES"
       disableMainThreadChecker = "YES"
       codeCoverageEnabled = "YES">
diff --git a/Reflection/Struct.hpp b/Reflection/Struct.hpp
index 67b76288e..a367d69a7 100644
--- a/Reflection/Struct.hpp
+++ b/Reflection/Struct.hpp
@@ -13,6 +13,7 @@
 #include <cstring>
 #include <cstddef>
 #include <cstdint>
+#include <mutex>
 #include <string>
 #include <typeindex>
 #include <typeinfo>
@@ -150,6 +151,13 @@ template <typename Type> Type get(const Struct &target, const std::string &name,
 
 template <typename Owner> class StructImpl: public Struct {
 public:
+	StructImpl<Owner>() {
+		static std::once_flag once;
+		std::call_once(once, [this] {
+			static_cast<Owner *>(this)->declare_fields();
+		});
+	}
+
 	/*!
 		@returns the value of type @c Type that is loaded from the offset registered for the field @c name.
 			It is the caller's responsibility to provide an appropriate type of data.
@@ -344,15 +352,6 @@ private:
 	};
 	static inline std::unordered_map<std::string, Field> contents_;
 	static inline std::unordered_map<std::string, std::vector<bool>> permitted_enum_values_;
-
-	// Ensure fields are declared at startup.
-	struct Declarer {
-		Declarer() {
-			Owner o;
-			o.declare_fields();
-		}
-	};
-	static Declarer declarer;
 };