Fix return value of sanitize when subformat is not readable

This is a fix on top of the previous issue fixed in
c917965b.

This was caught by "libFuzzer" testing.

src/hb-open-type-private.hh

@@ -165,6 +165,7 @@ struct hb_dispatch_context_t
   typedef Return return_t;
   template <typename T, typename F>
   inline bool may_dispatch (const T *obj, const F *format) { return true; }
+  static return_t no_dispatch_return_value (void) { return Context::default_return_value (); }
 };
 
 
@@ -203,6 +204,7 @@ struct hb_sanitize_context_t :
   template <typename T>
   inline return_t dispatch (const T &obj) { return obj.sanitize (this); }
   static return_t default_return_value (void) { return true; }
+  static return_t no_dispatch_return_value (void) { return false; }
   bool stop_sublookup_iteration (const return_t r) const { return !r; }
 
   inline void init (hb_blob_t *b)

src/hb-ot-layout-gpos-table.hh

@@ -548,7 +548,7 @@ struct SinglePos
   inline typename context_t::return_t dispatch (context_t *c) const
   {
     TRACE_DISPATCH (this, u.format);
-    if (unlikely (!c->may_dispatch (this, &u.format))) return_trace (c->default_return_value ());
+    if (unlikely (!c->may_dispatch (this, &u.format))) return_trace (c->no_dispatch_return_value ());
     switch (u.format) {
     case 1: return_trace (c->dispatch (u.format1));
     case 2: return_trace (c->dispatch (u.format2));
@@ -843,7 +843,7 @@ struct PairPos
   inline typename context_t::return_t dispatch (context_t *c) const
   {
     TRACE_DISPATCH (this, u.format);
-    if (unlikely (!c->may_dispatch (this, &u.format))) return_trace (c->default_return_value ());
+    if (unlikely (!c->may_dispatch (this, &u.format))) return_trace (c->no_dispatch_return_value ());
     switch (u.format) {
     case 1: return_trace (c->dispatch (u.format1));
     case 2: return_trace (c->dispatch (u.format2));
@@ -1024,7 +1024,7 @@ struct CursivePos
   inline typename context_t::return_t dispatch (context_t *c) const
   {
     TRACE_DISPATCH (this, u.format);
-    if (unlikely (!c->may_dispatch (this, &u.format))) return_trace (c->default_return_value ());
+    if (unlikely (!c->may_dispatch (this, &u.format))) return_trace (c->no_dispatch_return_value ());
     switch (u.format) {
     case 1: return_trace (c->dispatch (u.format1));
     default:return_trace (c->default_return_value ());
@@ -1120,7 +1120,7 @@ struct MarkBasePos
   inline typename context_t::return_t dispatch (context_t *c) const
   {
     TRACE_DISPATCH (this, u.format);
-    if (unlikely (!c->may_dispatch (this, &u.format))) return_trace (c->default_return_value ());
+    if (unlikely (!c->may_dispatch (this, &u.format))) return_trace (c->no_dispatch_return_value ());
     switch (u.format) {
     case 1: return_trace (c->dispatch (u.format1));
     default:return_trace (c->default_return_value ());
@@ -1238,7 +1238,7 @@ struct MarkLigPos
   inline typename context_t::return_t dispatch (context_t *c) const
   {
     TRACE_DISPATCH (this, u.format);
-    if (unlikely (!c->may_dispatch (this, &u.format))) return_trace (c->default_return_value ());
+    if (unlikely (!c->may_dispatch (this, &u.format))) return_trace (c->no_dispatch_return_value ());
     switch (u.format) {
     case 1: return_trace (c->dispatch (u.format1));
     default:return_trace (c->default_return_value ());
@@ -1353,7 +1353,7 @@ struct MarkMarkPos
   inline typename context_t::return_t dispatch (context_t *c) const
   {
     TRACE_DISPATCH (this, u.format);
-    if (unlikely (!c->may_dispatch (this, &u.format))) return_trace (c->default_return_value ());
+    if (unlikely (!c->may_dispatch (this, &u.format))) return_trace (c->no_dispatch_return_value ());
     switch (u.format) {
     case 1: return_trace (c->dispatch (u.format1));
     default:return_trace (c->default_return_value ());
@@ -1404,8 +1404,7 @@ struct PosLookupSubTable
   inline typename context_t::return_t dispatch (context_t *c, unsigned int lookup_type) const
   {
     TRACE_DISPATCH (this, lookup_type);
-    /* The sub_format passed to may_dispatch is unnecessary but harmless. */
-    if (unlikely (!c->may_dispatch (this, &u.sub_format))) return_trace (c->default_return_value ());
+    if (unlikely (!c->may_dispatch (this, &u.sub_format))) return_trace (c->no_dispatch_return_value ());
     switch (lookup_type) {
     case Single:		return_trace (u.single.dispatch (c));
     case Pair:			return_trace (u.pair.dispatch (c));

src/hb-ot-layout-gsub-table.hh

@@ -225,7 +225,7 @@ struct SingleSubst
   inline typename context_t::return_t dispatch (context_t *c) const
   {
     TRACE_DISPATCH (this, u.format);
-    if (unlikely (!c->may_dispatch (this, &u.format))) return_trace (c->default_return_value ());
+    if (unlikely (!c->may_dispatch (this, &u.format))) return_trace (c->no_dispatch_return_value ());
     switch (u.format) {
     case 1: return_trace (c->dispatch (u.format1));
     case 2: return_trace (c->dispatch (u.format2));
@@ -418,7 +418,7 @@ struct MultipleSubst
   inline typename context_t::return_t dispatch (context_t *c) const
   {
     TRACE_DISPATCH (this, u.format);
-    if (unlikely (!c->may_dispatch (this, &u.format))) return_trace (c->default_return_value ());
+    if (unlikely (!c->may_dispatch (this, &u.format))) return_trace (c->no_dispatch_return_value ());
     switch (u.format) {
     case 1: return_trace (c->dispatch (u.format1));
     default:return_trace (c->default_return_value ());
@@ -562,7 +562,7 @@ struct AlternateSubst
   inline typename context_t::return_t dispatch (context_t *c) const
   {
     TRACE_DISPATCH (this, u.format);
-    if (unlikely (!c->may_dispatch (this, &u.format))) return_trace (c->default_return_value ());
+    if (unlikely (!c->may_dispatch (this, &u.format))) return_trace (c->no_dispatch_return_value ());
     switch (u.format) {
     case 1: return_trace (c->dispatch (u.format1));
     default:return_trace (c->default_return_value ());
@@ -878,7 +878,7 @@ struct LigatureSubst
   inline typename context_t::return_t dispatch (context_t *c) const
   {
     TRACE_DISPATCH (this, u.format);
-    if (unlikely (!c->may_dispatch (this, &u.format))) return_trace (c->default_return_value ());
+    if (unlikely (!c->may_dispatch (this, &u.format))) return_trace (c->no_dispatch_return_value ());
     switch (u.format) {
     case 1: return_trace (c->dispatch (u.format1));
     default:return_trace (c->default_return_value ());
@@ -1035,7 +1035,7 @@ struct ReverseChainSingleSubst
   inline typename context_t::return_t dispatch (context_t *c) const
   {
     TRACE_DISPATCH (this, u.format);
-    if (unlikely (!c->may_dispatch (this, &u.format))) return_trace (c->default_return_value ());
+    if (unlikely (!c->may_dispatch (this, &u.format))) return_trace (c->no_dispatch_return_value ());
     switch (u.format) {
     case 1: return_trace (c->dispatch (u.format1));
     default:return_trace (c->default_return_value ());
@@ -1074,8 +1074,7 @@ struct SubstLookupSubTable
   inline typename context_t::return_t dispatch (context_t *c, unsigned int lookup_type) const
   {
     TRACE_DISPATCH (this, lookup_type);
-    /* The sub_format passed to may_dispatch is unnecessary but harmless. */
-    if (unlikely (!c->may_dispatch (this, &u.sub_format))) return_trace (c->default_return_value ());
+    if (unlikely (!c->may_dispatch (this, &u.sub_format))) return_trace (c->no_dispatch_return_value ());
     switch (lookup_type) {
     case Single:		return_trace (u.single.dispatch (c));
     case Multiple:		return_trace (u.multiple.dispatch (c));

src/hb-ot-layout-gsubgpos-private.hh

@@ -1505,7 +1505,7 @@ struct Context
   inline typename context_t::return_t dispatch (context_t *c) const
   {
     TRACE_DISPATCH (this, u.format);
-    if (unlikely (!c->may_dispatch (this, &u.format))) return_trace (c->default_return_value ());
+    if (unlikely (!c->may_dispatch (this, &u.format))) return_trace (c->no_dispatch_return_value ());
     switch (u.format) {
     case 1: return_trace (c->dispatch (u.format1));
     case 2: return_trace (c->dispatch (u.format2));
@@ -2120,7 +2120,7 @@ struct ChainContext
   inline typename context_t::return_t dispatch (context_t *c) const
   {
     TRACE_DISPATCH (this, u.format);
-    if (unlikely (!c->may_dispatch (this, &u.format))) return_trace (c->default_return_value ());
+    if (unlikely (!c->may_dispatch (this, &u.format))) return_trace (c->no_dispatch_return_value ());
     switch (u.format) {
     case 1: return_trace (c->dispatch (u.format1));
     case 2: return_trace (c->dispatch (u.format2));
@@ -2156,7 +2156,7 @@ struct ExtensionFormat1
   inline typename context_t::return_t dispatch (context_t *c) const
   {
     TRACE_DISPATCH (this, format);
-    if (unlikely (!c->may_dispatch (this, this))) return_trace (c->default_return_value ());
+    if (unlikely (!c->may_dispatch (this, this))) return_trace (c->no_dispatch_return_value ());
     return_trace (get_subtable<typename T::LookupSubTable> ().dispatch (c, get_type ()));
   }
 
@@ -2201,7 +2201,7 @@ struct Extension
   inline typename context_t::return_t dispatch (context_t *c) const
   {
     TRACE_DISPATCH (this, u.format);
-    if (unlikely (!c->may_dispatch (this, &u.format))) return_trace (c->default_return_value ());
+    if (unlikely (!c->may_dispatch (this, &u.format))) return_trace (c->no_dispatch_return_value ());
     switch (u.format) {
     case 1: return_trace (u.format1.dispatch (c));
     default:return_trace (c->default_return_value ());