diff --git a/libcxx/include/span b/libcxx/include/span
--- a/libcxx/include/span
+++ b/libcxx/include/span
@@ -218,7 +218,7 @@
     static constexpr size_type extent = _Extent;
 
 // [span.cons], span constructors, copy, assignment, and destructor
-    template <size_t _Sz = _Extent, enable_if_t<_Sz == 0, nullptr_t> = nullptr>
+    template <size_t _Sz = _Extent> requires(_Sz == 0)
     _LIBCPP_INLINE_VISIBILITY constexpr span() noexcept : __data{nullptr} {}
 
     constexpr span           (const span&) noexcept = default;
@@ -231,46 +231,40 @@
 
     _LIBCPP_INLINE_VISIBILITY constexpr span(element_type (&__arr)[_Extent])          noexcept : __data{__arr} {}
 
-    template <class _OtherElementType,
-              enable_if_t<is_convertible_v<_OtherElementType(*)[], element_type (*)[]>, nullptr_t> = nullptr>
+    template <class _OtherElementType> requires(is_convertible_v<_OtherElementType(*)[], element_type (*)[]>)
     _LIBCPP_INLINE_VISIBILITY
     constexpr span(array<_OtherElementType, _Extent>& __arr) noexcept : __data{__arr.data()} {}
 
-    template <class _OtherElementType,
-              enable_if_t<is_convertible_v<const _OtherElementType(*)[], element_type (*)[]>, nullptr_t> = nullptr>
+    template <class _OtherElementType> requires(is_convertible_v<const _OtherElementType(*)[], element_type (*)[]>)
     _LIBCPP_INLINE_VISIBILITY
     constexpr span(const array<_OtherElementType, _Extent>& __arr) noexcept : __data{__arr.data()} {}
 
     template <class _Container>
     _LIBCPP_INLINE_VISIBILITY
-        constexpr explicit span(      _Container& __c,
-            enable_if_t<__is_span_compatible_container<_Container, _Tp>::value, nullptr_t> = nullptr)
+        constexpr explicit span(      _Container& __c)
+            requires(__is_span_compatible_container<_Container, _Tp>::value)
         : __data{_VSTD::data(__c)} {
             _LIBCPP_ASSERT(_Extent == _VSTD::size(__c), "size mismatch in span's constructor (range)");
         }
 
     template <class _Container>
     _LIBCPP_INLINE_VISIBILITY
-        constexpr explicit span(const _Container& __c,
-            enable_if_t<__is_span_compatible_container<const _Container, _Tp>::value, nullptr_t> = nullptr)
+        constexpr explicit span(const _Container& __c)
+            requires(__is_span_compatible_container<const _Container, _Tp>::value)
         : __data{_VSTD::data(__c)} {
             _LIBCPP_ASSERT(_Extent == _VSTD::size(__c), "size mismatch in span's constructor (range)");
         }
 
     template <class _OtherElementType>
+            requires(is_convertible_v<_OtherElementType(*)[], element_type (*)[]>)
     _LIBCPP_INLINE_VISIBILITY
-        constexpr span(const span<_OtherElementType, _Extent>& __other,
-                       enable_if_t<
-                          is_convertible_v<_OtherElementType(*)[], element_type (*)[]>,
-                          nullptr_t> = nullptr)
+        constexpr span(const span<_OtherElementType, _Extent>& __other)
         : __data{__other.data()} {}
 
     template <class _OtherElementType>
+            requires(is_convertible_v<_OtherElementType(*)[], element_type (*)[]>)
     _LIBCPP_INLINE_VISIBILITY
-        constexpr explicit span(const span<_OtherElementType, dynamic_extent>& __other,
-                       enable_if_t<
-                          is_convertible_v<_OtherElementType(*)[], element_type (*)[]>,
-                          nullptr_t> = nullptr) noexcept
+        constexpr explicit span(const span<_OtherElementType, dynamic_extent>& __other) noexcept
         : __data{__other.data()} { _LIBCPP_ASSERT(_Extent == __other.size(), "size mismatch in span's constructor (other span)"); }
 
 
@@ -409,35 +403,33 @@
     _LIBCPP_INLINE_VISIBILITY
     constexpr span(element_type (&__arr)[_Sz])          noexcept : __data{__arr}, __size{_Sz} {}
 
-    template <class _OtherElementType, size_t _Sz,
-              enable_if_t<is_convertible_v<_OtherElementType(*)[], element_type (*)[]>, nullptr_t> = nullptr>
+    template <class _OtherElementType, size_t _Sz>
+              requires(is_convertible_v<_OtherElementType(*)[], element_type (*)[]>)
     _LIBCPP_INLINE_VISIBILITY
     constexpr span(array<_OtherElementType, _Sz>& __arr) noexcept : __data{__arr.data()}, __size{_Sz} {}
 
-    template <class _OtherElementType, size_t _Sz,
-              enable_if_t<is_convertible_v<const _OtherElementType(*)[], element_type (*)[]>, nullptr_t> = nullptr>
+    template <class _OtherElementType, size_t _Sz>
+              requires(is_convertible_v<const _OtherElementType(*)[], element_type (*)[]>)
     _LIBCPP_INLINE_VISIBILITY
     constexpr span(const array<_OtherElementType, _Sz>& __arr) noexcept : __data{__arr.data()}, __size{_Sz} {}
 
     template <class _Container>
     _LIBCPP_INLINE_VISIBILITY
-        constexpr span(      _Container& __c,
-            enable_if_t<__is_span_compatible_container<_Container, _Tp>::value, nullptr_t> = nullptr)
+        constexpr span(      _Container& __c)
+            requires(__is_span_compatible_container<_Container, _Tp>::value)
         : __data{_VSTD::data(__c)}, __size{(size_type) _VSTD::size(__c)} {}
 
     template <class _Container>
     _LIBCPP_INLINE_VISIBILITY
-        constexpr span(const _Container& __c,
-            enable_if_t<__is_span_compatible_container<const _Container, _Tp>::value, nullptr_t> = nullptr)
+        constexpr span(const _Container& __c)
+            requires(__is_span_compatible_container<const _Container, _Tp>::value)
         : __data{_VSTD::data(__c)}, __size{(size_type) _VSTD::size(__c)} {}
 
 
     template <class _OtherElementType, size_t _OtherExtent>
+            requires(is_convertible_v<_OtherElementType(*)[], element_type (*)[]>)
     _LIBCPP_INLINE_VISIBILITY
-        constexpr span(const span<_OtherElementType, _OtherExtent>& __other,
-                       enable_if_t<
-                          is_convertible_v<_OtherElementType(*)[], element_type (*)[]>,
-                          nullptr_t> = nullptr) noexcept
+        constexpr span(const span<_OtherElementType, _OtherExtent>& __other) noexcept
         : __data{__other.data()}, __size{__other.size()} {}
 
 //    ~span() noexcept = default;
@@ -550,10 +542,10 @@
 -> decltype(__s.__as_bytes())
 { return    __s.__as_bytes(); }
 
-template <class _Tp, size_t _Extent>
+template <class _Tp, size_t _Extent> requires(!is_const_v<_Tp>)
 _LIBCPP_INLINE_VISIBILITY
 auto as_writable_bytes(span<_Tp, _Extent> __s) noexcept
--> enable_if_t<!is_const_v<_Tp>, decltype(__s.__as_writable_bytes())>
+-> decltype(__s.__as_writable_bytes())
 { return __s.__as_writable_bytes(); }
 
 //  Deduction guides