diff --git a/src/hb-open-type-private.hh b/src/hb-open-type-private.hh
index b585e1873..15198b176 100644
--- a/src/hb-open-type-private.hh
+++ b/src/hb-open-type-private.hh
@@ -37,11 +37,6 @@
 namespace OT {
 
 
-/*
- * Void!
- */
-typedef struct {} void_t;
-
 
 /*
  * Casts
diff --git a/src/hb-ot-layout-gsubgpos-private.hh b/src/hb-ot-layout-gsubgpos-private.hh
index 55ea9ca5c..608ed5ae8 100644
--- a/src/hb-ot-layout-gsubgpos-private.hh
+++ b/src/hb-ot-layout-gsubgpos-private.hh
@@ -50,7 +50,7 @@ struct hb_closure_context_t
   typedef return_t (*recurse_func_t) (hb_closure_context_t *c, unsigned int lookup_index);
   template <typename T>
   inline return_t process (const T &obj) { obj.closure (this); return void_t (); }
-  static const return_t default_return_value (void) { return return_t (); }
+  static return_t default_return_value (void) { return return_t (); }
   bool stop_sublookup_iteration (const return_t r) const { return false; }
   return_t recurse (unsigned int lookup_index)
   {
@@ -82,7 +82,6 @@ struct hb_closure_context_t
 
 
 
-/* TODO Add TRACE_RETURN annotation to gsub. */
 #ifndef HB_DEBUG_WOULD_APPLY
 #define HB_DEBUG_WOULD_APPLY (HB_DEBUG+0)
 #endif
@@ -119,7 +118,6 @@ struct hb_would_apply_context_t
 
 
 
-/* TODO Add TRACE_RETURN annotation to gsub. */
 #ifndef HB_DEBUG_COLLECT_GLYPHS
 #define HB_DEBUG_COLLECT_GLYPHS (HB_DEBUG+0)
 #endif
@@ -133,7 +131,7 @@ struct hb_collect_glyphs_context_t
   typedef void_t return_t;
   template <typename T>
   inline return_t process (const T &obj) { obj.collect_glyphs (this); return void_t (); }
-  static const return_t default_return_value (void) { return return_t (); }
+  static return_t default_return_value (void) { return return_t (); }
   bool stop_iteration (const return_t r) const { return false; }
   return_t recurse (unsigned int lookup_index)
   {
@@ -170,7 +168,7 @@ struct hb_get_coverage_context_t
   typedef const Coverage &return_t;
   template <typename T>
   inline return_t process (const T &obj) { return obj.get_coverage (); }
-  static return_t default_return_value (void) { return Null(Coverage); }
+  static const return_t default_return_value (void) { return Null(Coverage); }
   bool stop_sublookup_iteration (const return_t r) const { return true; /* Unused */ }
   return_t recurse (unsigned int lookup_index)
   { return default_return_value (); }
@@ -194,7 +192,7 @@ struct hb_apply_context_t
   typedef return_t (*recurse_func_t) (hb_apply_context_t *c, unsigned int lookup_index);
   template <typename T>
   inline return_t process (const T &obj) { return obj.apply (this); }
-  static const return_t default_return_value (void) { return false; }
+  static return_t default_return_value (void) { return false; }
   bool stop_sublookup_iteration (const return_t r) const { return r; }
   return_t recurse (unsigned int lookup_index)
   {
diff --git a/src/hb-private.hh b/src/hb-private.hh
index 5c5359190..16c356217 100644
--- a/src/hb-private.hh
+++ b/src/hb-private.hh
@@ -62,6 +62,10 @@
 #endif
 
 
+/* Void! */
+typedef struct {} void_t;
+
+
 /* Basics */
 
 
@@ -656,6 +660,24 @@ _hb_debug_msg<0> (const char *what HB_UNUSED,
 #define DEBUG_MSG_FUNC(WHAT, OBJ, ...)				_hb_debug_msg<HB_DEBUG_##WHAT> (#WHAT, (OBJ), HB_FUNC, false, 0, 0, __VA_ARGS__)
 
 
+/*
+ * Printer
+ */
+
+template <typename T>
+struct hb_printer_t {};
+
+template <>
+struct hb_printer_t<bool> {
+  const char *print (bool v) { return v ? "true" : "false"; }
+};
+
+template <>
+struct hb_printer_t<void_t> {
+  const char *print (void_t v) { return ""; }
+};
+
+
 /*
  * Trace
  */
@@ -687,14 +709,15 @@ struct hb_auto_trace_t {
     if (plevel) --*plevel;
   }
 
-  inline bool ret (bool v, unsigned int line = 0)
+  template <typename T>
+  inline T ret (T v, unsigned int line = 0)
   {
     if (unlikely (returned)) {
       fprintf (stderr, "OUCH, double calls to TRACE_RETURN.  This is a bug, please report.\n");
       return v;
     }
 
-    _hb_debug_msg<max_level> (what, obj, NULL, true, plevel ? *plevel : 1, -1, "return %s (line %d)", v ? "true" : "false", line);
+    _hb_debug_msg<max_level> (what, obj, NULL, true, plevel ? *plevel : 1, -1, "return %s (line %d)", hb_printer_t<bool>().print (v), line);
     if (plevel) --*plevel;
     plevel = NULL;
     returned = true;