diff --git a/src/hb-iter.hh b/src/hb-iter.hh
index 7358256c80493f065506e810fb227db71cd552d5..5ad839e2aa393cf578103d7e936b528ebac4df80 100644
--- a/src/hb-iter.hh
+++ b/src/hb-iter.hh
@@ -281,11 +281,11 @@ struct hb_is_iterator_of { enum {
 /* Range-based 'for' for iterables. */
 
 template <typename Iterable,
-	  hb_enable_if (hb_is_iterable (Iterable))>
+	  hb_requires (hb_is_iterable (Iterable))>
 static inline auto begin (Iterable&& iterable) HB_AUTO_RETURN (hb_iter (iterable).begin ())
 
 template <typename Iterable,
-	  hb_enable_if (hb_is_iterable (Iterable))>
+	  hb_requires (hb_is_iterable (Iterable))>
 static inline auto end (Iterable&& iterable) HB_AUTO_RETURN (hb_iter (iterable).end ())
 
 /* begin()/end() are NOT looked up non-ADL.  So each namespace must declare them.
@@ -293,11 +293,11 @@ static inline auto end (Iterable&& iterable) HB_AUTO_RETURN (hb_iter (iterable).
 namespace OT {
 
 template <typename Iterable,
-	  hb_enable_if (hb_is_iterable (Iterable))>
+	  hb_requires (hb_is_iterable (Iterable))>
 static inline auto begin (Iterable&& iterable) HB_AUTO_RETURN (hb_iter (iterable).begin ())
 
 template <typename Iterable,
-	  hb_enable_if (hb_is_iterable (Iterable))>
+	  hb_requires (hb_is_iterable (Iterable))>
 static inline auto end (Iterable&& iterable) HB_AUTO_RETURN (hb_iter (iterable).end ())
 
 }
@@ -308,14 +308,14 @@ static inline auto end (Iterable&& iterable) HB_AUTO_RETURN (hb_iter (iterable).
  */
 
 template <typename Lhs, typename Rhs,
-	  hb_enable_if (hb_is_iterator (Lhs))>
+	  hb_requires (hb_is_iterator (Lhs))>
 static inline auto
 operator | (Lhs lhs, const Rhs &rhs) HB_AUTO_RETURN (rhs (lhs))
 
 /* hb_map(), hb_filter(), hb_reduce() */
 
 template <typename Iter, typename Proj,
-	 hb_enable_if (hb_is_iterator (Iter))>
+	 hb_requires (hb_is_iterator (Iter))>
 struct hb_map_iter_t :
   hb_iter_t<hb_map_iter_t<Iter, Proj>,
 	    decltype (hb_declval (Proj) (hb_declval (typename Iter::item_t)))>
@@ -347,7 +347,7 @@ struct hb_map_iter_factory_t
   hb_map_iter_factory_t (Proj f) : f (f) {}
 
   template <typename Iter,
-	    hb_enable_if (hb_is_iterator (Iter))>
+	    hb_requires (hb_is_iterator (Iter))>
   hb_map_iter_t<Iter, Proj>
   operator () (Iter it) const
   { return hb_map_iter_t<Iter, Proj> (it, f); }
@@ -365,7 +365,7 @@ struct
 HB_FUNCOBJ (hb_map);
 
 template <typename Iter, typename Pred, typename Proj,
-	 hb_enable_if (hb_is_iterator (Iter))>
+	 hb_requires (hb_is_iterator (Iter))>
 struct hb_filter_iter_t :
   hb_iter_with_fallback_t<hb_filter_iter_t<Iter, Pred, Proj>,
 			  typename Iter::item_t>
@@ -394,7 +394,7 @@ struct hb_filter_iter_factory_t
   hb_filter_iter_factory_t (Pred p, Proj f) : p (p), f (f) {}
 
   template <typename Iter,
-	    hb_enable_if (hb_is_iterator (Iter))>
+	    hb_requires (hb_is_iterator (Iter))>
   hb_filter_iter_t<Iter, Pred, Proj>
   operator () (Iter it) const
   { return hb_filter_iter_t<Iter, Pred, Proj> (it, p, f); }
@@ -419,7 +419,7 @@ struct hb_reduce_t
   hb_reduce_t (Redu r, InitT init_value) : r (r), init_value (init_value) {}
 
   template <typename Iter,
-	    hb_enable_if (hb_is_iterator (Iter)),
+	    hb_requires (hb_is_iterator (Iter)),
 	    typename AccuT = decltype (hb_declval (Redu) (hb_declval (InitT), hb_declval (typename Iter::item_t)))>
   AccuT
   operator () (Iter it) const
@@ -480,7 +480,7 @@ struct hb_zip_iter_t :
 struct
 {
   template <typename A, typename B,
-	    hb_enable_if (hb_is_iterable (A) && hb_is_iterable (B))>
+	    hb_requires (hb_is_iterable (A) && hb_is_iterable (B))>
   hb_zip_iter_t<hb_iter_t (A), hb_iter_t (B)>
   operator () (A& a, B &b) const
   { return hb_zip_iter_t<hb_iter_t (A), hb_iter_t (B)> (hb_iter (a), hb_iter (b)); }
@@ -490,7 +490,7 @@ HB_FUNCOBJ (hb_zip);
 /* hb_enumerate */
 
 template <typename Iter,
-	 hb_enable_if (hb_is_iterator (Iter))>
+	 hb_requires (hb_is_iterator (Iter))>
 struct hb_enumerate_iter_t :
   hb_iter_t<hb_enumerate_iter_t<Iter>,
 	    hb_pair_t<unsigned, typename Iter::item_t>>
@@ -527,7 +527,7 @@ struct hb_enumerate_iter_t :
 struct
 {
   template <typename Iterable,
-	    hb_enable_if (hb_is_iterable (Iterable))>
+	    hb_requires (hb_is_iterable (Iterable))>
   hb_enumerate_iter_t<hb_iter_t (Iterable)>
   operator () (Iterable& it) const
   { return hb_enumerate_iter_t<hb_iter_t (Iterable)> (hb_iter (it)); }
@@ -542,9 +542,8 @@ struct hb_apply_t
   hb_apply_t (Appl a) : a (a) {}
 
   template <typename Iter,
-	    hb_enable_if (hb_is_iterator (Iter))>
-  void
-  operator () (Iter it) const
+	    hb_requires (hb_is_iterator (Iter))>
+  void operator () (Iter it) const
   {
     for (; it; ++it)
       (void) hb_invoke (a, *it);
@@ -573,9 +572,8 @@ struct hb_sink_t
   hb_sink_t (Sink&& s) : s (s) {}
 
   template <typename Iter,
-	    hb_enable_if (hb_is_iterator (Iter))>
-  void
-  operator () (Iter it) const
+	    hb_requires (hb_is_iterator (Iter))>
+  void operator () (Iter it) const
   {
     for (; it; ++it)
       s << *it;
@@ -601,9 +599,8 @@ HB_FUNCOBJ (hb_sink);
 struct
 {
   template <typename Iter,
-	    hb_enable_if (hb_is_iterator (Iter))>
-  void
-  operator () (Iter it) const
+	    hb_requires (hb_is_iterator (Iter))>
+  void operator () (Iter it) const
   {
     for (; it; ++it)
       (void) *it;
@@ -619,9 +616,8 @@ struct hb_unzip_t
   hb_unzip_t (Sink1&& s1, Sink2&& s2) : s1 (s1), s2 (s2) {}
 
   template <typename Iter,
-	    hb_enable_if (hb_is_iterator (Iter))>
-  void
-  operator () (Iter it) const
+	    hb_requires (hb_is_iterator (Iter))>
+  void operator () (Iter it) const
   {
     for (; it; ++it)
     {
@@ -653,9 +649,8 @@ HB_FUNCOBJ (hb_unzip);
 struct
 {
   template <typename Iterable,
-	    hb_enable_if (hb_is_iterable (Iterable))>
-  bool
-  operator () (Iterable&& c) const
+	    hb_requires (hb_is_iterable (Iterable))>
+  bool operator () (Iterable&& c) const
   {
     for (auto it = hb_iter (c); it; ++it)
       if (!*it)
@@ -667,9 +662,8 @@ HB_FUNCOBJ (hb_all);
 struct
 {
   template <typename Iterable,
-	    hb_enable_if (hb_is_iterable (Iterable))>
-  bool
-  operator () (Iterable&& c) const
+	    hb_requires (hb_is_iterable (Iterable))>
+  bool operator () (Iterable&& c) const
   {
     for (auto it = hb_iter (c); it; ++it)
       if (*it)
@@ -681,9 +675,8 @@ HB_FUNCOBJ (hb_any);
 struct
 {
   template <typename Iterable,
-	    hb_enable_if (hb_is_iterable (Iterable))>
-  bool
-  operator () (Iterable&& c) const
+	    hb_requires (hb_is_iterable (Iterable))>
+  bool operator () (Iterable&& c) const
   {
     for (auto it = hb_iter (c); it; ++it)
       if (*it)
@@ -698,7 +691,7 @@ HB_FUNCOBJ (hb_none);
  */
 
 template <typename C, typename V,
-	  hb_enable_if (hb_is_iterable (C))>
+	  hb_requires (hb_is_iterable (C))>
 inline void
 hb_fill (C& c, const V &v)
 {
diff --git a/src/hb-open-type.hh b/src/hb-open-type.hh
index e2e562300d6becf0b468867d4f99f4641df1d241..5c015af3e775ed67c28b4c52b223ea7595b0a602 100644
--- a/src/hb-open-type.hh
+++ b/src/hb-open-type.hh
@@ -419,7 +419,7 @@ struct UnsizedArrayOf
     return_trace (true);
   }
   template <typename Iterator,
-	    hb_enable_if (hb_is_iterator_of (Iterator, const Type))>
+	    hb_requires (hb_is_iterator_of (Iterator, const Type))>
   bool serialize (hb_serialize_context_t *c, Iterator items)
   {
     TRACE_SERIALIZE (this);
@@ -600,7 +600,7 @@ struct ArrayOf
     return_trace (true);
   }
   template <typename Iterator,
-	    hb_enable_if (hb_is_iterator_of (Iterator, const Type))>
+	    hb_requires (hb_is_iterator_of (Iterator, const Type))>
   bool serialize (hb_serialize_context_t *c, Iterator items)
   {
     TRACE_SERIALIZE (this);
@@ -880,7 +880,7 @@ struct SortedArrayOf : ArrayOf<Type, LenType>
     return_trace (ret);
   }
   template <typename Iterator,
-	    hb_enable_if (hb_is_sorted_iterator_of (Iterator, const Type))>
+	    hb_requires (hb_is_sorted_iterator_of (Iterator, const Type))>
   bool serialize (hb_serialize_context_t *c, Iterator items)
   {
     TRACE_SERIALIZE (this);
diff --git a/src/hb-ot-layout-common.hh b/src/hb-ot-layout-common.hh
index 17a03c85f5c4ab25c7e2292991e24cb0ed7bf534..05f84d2544091eb913c03f2ee9c767a5966352cc 100644
--- a/src/hb-ot-layout-common.hh
+++ b/src/hb-ot-layout-common.hh
@@ -797,7 +797,7 @@ struct CoverageFormat1
   }
 
   template <typename Iterator,
-	    hb_enable_if (hb_is_sorted_iterator_of (Iterator, const GlyphID))>
+	    hb_requires (hb_is_sorted_iterator_of (Iterator, const GlyphID))>
   bool serialize (hb_serialize_context_t *c, Iterator glyphs)
   {
     TRACE_SERIALIZE (this);
@@ -866,7 +866,7 @@ struct CoverageFormat2
   }
 
   template <typename Iterator,
-	    hb_enable_if (hb_is_sorted_iterator_of (Iterator, const GlyphID))>
+	    hb_requires (hb_is_sorted_iterator_of (Iterator, const GlyphID))>
   bool serialize (hb_serialize_context_t *c, Iterator glyphs)
   {
     TRACE_SERIALIZE (this);
@@ -1030,7 +1030,7 @@ struct Coverage
   }
 
   template <typename Iterator,
-	    hb_enable_if (hb_is_sorted_iterator_of (Iterator, const GlyphID))>
+	    hb_requires (hb_is_sorted_iterator_of (Iterator, const GlyphID))>
   bool serialize (hb_serialize_context_t *c, Iterator glyphs)
   {
     TRACE_SERIALIZE (this);
diff --git a/src/test-iter.cc b/src/test-iter.cc
index afbcbe9b51052b7d1c0ae3c9e58040671ba65f4a..e8c6bda6b2f71fa101e50b981bb8abbd0ecf1e27 100644
--- a/src/test-iter.cc
+++ b/src/test-iter.cc
@@ -65,7 +65,7 @@ struct some_array_t
 
 
 template <typename Iter,
-	  hb_enable_if (hb_is_iterator (Iter))>
+	  hb_requires (hb_is_iterator (Iter))>
 static void
 test_iterator_non_default_constructable (Iter it)
 {
@@ -92,7 +92,7 @@ test_iterator_non_default_constructable (Iter it)
 }
 
 template <typename Iter,
-	  hb_enable_if (hb_is_iterator (Iter))>
+	  hb_requires (hb_is_iterator (Iter))>
 static void
 test_iterator (Iter it)
 {
@@ -103,7 +103,7 @@ test_iterator (Iter it)
 }
 
 template <typename Iterable,
-	  hb_enable_if (hb_is_iterable (Iterable))>
+	  hb_requires (hb_is_iterable (Iterable))>
 static void
 test_iterable (const Iterable &lst = Null(Iterable))
 {