diff --git a/src/hb-ot-layout-gsub-table.hh b/src/hb-ot-layout-gsub-table.hh
index 641bf6182..11c9bf19e 100644
--- a/src/hb-ot-layout-gsub-table.hh
+++ b/src/hb-ot-layout-gsub-table.hh
@@ -643,12 +643,9 @@ struct AlternateSubstFormat1
   { return c->len == 1 && (this+coverage).get_coverage (c->glyphs[0]) != NOT_COVERED; }
 
   unsigned
-  get_glyph_alternates (hb_codepoint_t  gid,
-			unsigned        start_offset,
-			unsigned       *alternate_count  /* IN/OUT.  May be NULL. */,
-			hb_codepoint_t *alternate_glyphs /* OUT.     May be NULL. */) const
-  { return (this+alternateSet[(this+coverage).get_coverage (gid)])
-	   .get_alternates (start_offset, alternate_count, alternate_glyphs); }
+  get_glyph_alternates (hb_get_glyph_alternates_context_t *c) const
+  { return (this+alternateSet[(this+coverage).get_coverage (c->gid)])
+	   .get_alternates (c->start_offset, c->alternate_count, c->alternate_glyphs); }
 
   bool apply (hb_ot_apply_context_t *c) const
   {
@@ -737,18 +734,6 @@ struct AlternateSubst
     }
   }
 
-  unsigned
-  get_glyph_alternates (hb_codepoint_t  gid,
-			unsigned        start_offset,
-			unsigned       *alternate_count  /* IN/OUT.  May be NULL. */,
-			hb_codepoint_t *alternate_glyphs /* OUT.     May be NULL. */) const
-  {
-    switch (u.format) {
-    case 1: return u.format1.get_glyph_alternates (gid, start_offset, alternate_count, alternate_glyphs);
-    default:assert (0); return 0;
-    }
-  }
-
   template <typename context_t, typename ...Ts>
   typename context_t::return_t dispatch (context_t *c, Ts&&... ds) const
   {
@@ -1497,26 +1482,6 @@ struct SubstLookup : Lookup
 			     substitute_glyphs_list));
   }
 
-  unsigned
-  get_glyph_alternates (hb_codepoint_t  gid,
-			unsigned        start_offset,
-			unsigned       *alternate_count  /* IN/OUT.  May be NULL. */,
-			hb_codepoint_t *alternate_glyphs /* OUT.     May be NULL. */) const
-  {
-    if (get_type () == SubTable::Alternate)
-    {
-      unsigned size = get_subtable_count ();
-      for (unsigned i = 0; i < size; ++i)
-      {
-	const AlternateSubst &subtable = get_subtable (i).u.alternate;
-	auto ret = subtable.get_glyph_alternates (gid, start_offset, alternate_count, alternate_glyphs);
-	if (ret) return (ret);
-      }
-    }
-    if (alternate_count) *alternate_count = 0;
-    return 0;
-  }
-
   bool serialize_alternate (hb_serialize_context_t *c,
 			    uint32_t lookup_props,
 			    hb_sorted_array_t<const HBGlyphID> glyphs,
diff --git a/src/hb-ot-layout-gsubgpos.hh b/src/hb-ot-layout-gsubgpos.hh
index 0026f83a5..75ef76a4b 100644
--- a/src/hb-ot-layout-gsubgpos.hh
+++ b/src/hb-ot-layout-gsubgpos.hh
@@ -227,6 +227,45 @@ struct hb_would_apply_context_t :
 			      debug_depth (0) {}
 };
 
+struct hb_get_glyph_alternates_context_t :
+       hb_dispatch_context_t<hb_get_glyph_alternates_context_t, unsigned, 0>
+{
+  const char *get_name () { return "GET_GLYPH_ALTERNATES"; }
+  static return_t default_return_value () { return 0; }
+  bool stop_sublookup_iteration (return_t r) const { return r; }
+
+  hb_face_t *face;
+  hb_codepoint_t gid;
+  unsigned        start_offset;
+  unsigned       *alternate_count;
+  hb_codepoint_t *alternate_glyphs;
+  unsigned int debug_depth;
+
+  hb_get_glyph_alternates_context_t (hb_face_t *face_,
+				     hb_codepoint_t gid_,
+				     unsigned        start_offset_,
+				     unsigned       *alternate_count_,
+				     hb_codepoint_t *alternate_glyphs_) :
+				       face (face_),
+				       gid (gid_),
+				       start_offset (start_offset_),
+				       alternate_count (alternate_count_),
+				       alternate_glyphs (alternate_glyphs_),
+				       debug_depth (0) {}
+
+  private:
+  template <typename T> auto
+  _dispatch (const T &obj, hb_priority<1>) HB_AUTO_RETURN
+  ( obj.get_glyph_alternates (this) )
+  template <typename T> auto
+  _dispatch (const T &obj, hb_priority<0>) HB_AUTO_RETURN
+  ( default_return_value () )
+  public:
+  template <typename T> auto
+  dispatch (const T &obj) HB_AUTO_RETURN
+  ( _dispatch (obj, hb_prioritize) )
+};
+
 
 struct hb_collect_glyphs_context_t :
        hb_dispatch_context_t<hb_collect_glyphs_context_t, hb_empty_t, 0>
diff --git a/src/hb-ot-layout.cc b/src/hb-ot-layout.cc
index 01587f1df..c042ce9cc 100644
--- a/src/hb-ot-layout.cc
+++ b/src/hb-ot-layout.cc
@@ -1994,8 +1994,11 @@ hb_ot_layout_lookup_get_glyph_alternates (hb_face_t      *face,
 					  unsigned       *alternate_count  /* IN/OUT.  May be NULL. */,
 					  hb_codepoint_t *alternate_glyphs /* OUT.     May be NULL. */)
 {
+  OT::hb_get_glyph_alternates_context_t c (face, glyph, start_offset, alternate_count, alternate_glyphs);
   const OT::SubstLookup &lookup = face->table.GSUB->table->get_lookup (lookup_index);
-  return lookup.get_glyph_alternates (glyph, start_offset, alternate_count, alternate_glyphs);
+  auto ret = lookup.dispatch (&c);
+  if (!ret && alternate_count) *alternate_count = 0;
+  return ret;
 }
 
 #endif