Diff to get Zoe's suggestion for bind_back

bind_back.diff

diff --git a/libcxx/include/__functional/bind_back.h b/libcxx/include/__functional/bind_back.h
index fd4d0ea75ade..110cab61f8d6 100644
--- a/libcxx/include/__functional/bind_back.h
+++ b/libcxx/include/__functional/bind_back.h
@@ -26,36 +26,22 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 
 #if _LIBCPP_STD_VER > 17
 
-// Passes the provided arguments to std::invoke in the order specified by the given index_sequence.
-template <size_t ..._Ip, class ..._Args, class _AsTuple = tuple<_Args&&...>>
-_LIBCPP_HIDE_FROM_ABI
-constexpr auto __invoke_shuffle(index_sequence<_Ip...>, _Args&& ...__args)
-    noexcept(is_nothrow_invocable_v<tuple_element_t<_Ip, _AsTuple>...>)
-    -> typename invoke_result<tuple_element_t<_Ip, _AsTuple>...>::type
-{
-    _AsTuple __as_tuple = _VSTD::forward_as_tuple(_VSTD::forward<_Args>(__args)...);
-    return _VSTD::invoke(_VSTD::forward<tuple_element_t<_Ip, _AsTuple>>(_VSTD::get<_Ip>(__as_tuple))...);
-}
+template <size_t _NBound, class = make_index_sequence<_NBound>>
+struct __bind_back_op;
 
-template <size_t _NBound>
-struct __bind_back_op {
-    template <class _Fn, class ..._Args,
-        size_t _NUnbound = sizeof...(_Args) - _NBound,
-        class _Sequence = __join_integer_sequences<
-            index_sequence<0>,                                                    // [0, 1) = the function object to invoke
-            __shift_index_sequence_by<_NBound+1, make_index_sequence<_NUnbound>>, // [_NBound+1, _NUnbound) = the passed arguments, which go first
-            __shift_index_sequence_by<1, make_index_sequence<_NBound>>            // [1, _NBound) = the bound arguments, which go last
-        >>
+template <size_t _NBound, size_t ..._Ip>
+struct __bind_back_op<_NBound, index_sequence<_Ip...>> {
+    template <class _Fn, class _Bound, class ..._Args>
     _LIBCPP_HIDE_FROM_ABI
-    static constexpr auto __call(_Fn&& __f, _Args&& ...__args)
-    noexcept(noexcept(_VSTD::__invoke_shuffle(_Sequence{}, _VSTD::forward<_Fn>(__f), _VSTD::forward<_Args>(__args)...)))
-    -> decltype(      _VSTD::__invoke_shuffle(_Sequence{}, _VSTD::forward<_Fn>(__f), _VSTD::forward<_Args>(__args)...))
-    { return          _VSTD::__invoke_shuffle(_Sequence{}, _VSTD::forward<_Fn>(__f), _VSTD::forward<_Args>(__args)...); }
+    static constexpr auto __call(_Fn&& __f, _Bound&& __bound, _Args&& ...__args)
+    noexcept(noexcept(_VSTD::invoke(_VSTD::forward<_Fn>(__f), _VSTD::forward<_Args>(__args)..., _VSTD::get<_Ip>(_VSTD::forward<_Bound>(__bound))...)))
+    -> decltype(      _VSTD::invoke(_VSTD::forward<_Fn>(__f), _VSTD::forward<_Args>(__args)..., _VSTD::get<_Ip>(_VSTD::forward<_Bound>(__bound))...))
+    { return          _VSTD::invoke(_VSTD::forward<_Fn>(__f), _VSTD::forward<_Args>(__args)..., _VSTD::get<_Ip>(_VSTD::forward<_Bound>(__bound))...); }
 };
 
-template <class _Fn, class ..._BoundArgs>
-struct __bind_back_t : __perfect_forward<__bind_back_op<sizeof...(_BoundArgs)>, _Fn, _BoundArgs...> {
-    using __perfect_forward<__bind_back_op<sizeof...(_BoundArgs)>, _Fn, _BoundArgs...>::__perfect_forward;
+template <class _Fn, class _BoundArgs>
+struct __bind_back_t : __perfect_forward<__bind_back_op<tuple_size_v<_BoundArgs>>, _Fn, _BoundArgs> {
+    using __perfect_forward<__bind_back_op<tuple_size_v<_BoundArgs>>, _Fn, _BoundArgs>::__perfect_forward;
 };
 
 template <class _Fn, class ..._Args, class = _EnableIf<
@@ -68,9 +54,9 @@ template <class _Fn, class ..._Args, class = _EnableIf<
 >>
 _LIBCPP_HIDE_FROM_ABI
 constexpr auto __bind_back(_Fn&& __f, _Args&&... __args)
-noexcept(noexcept(__bind_back_t<decay_t<_Fn>, decay_t<_Args>...>(_VSTD::forward<_Fn>(__f), _VSTD::forward<_Args>(__args)...)))
--> decltype(      __bind_back_t<decay_t<_Fn>, decay_t<_Args>...>(_VSTD::forward<_Fn>(__f), _VSTD::forward<_Args>(__args)...))
-{ return          __bind_back_t<decay_t<_Fn>, decay_t<_Args>...>(_VSTD::forward<_Fn>(__f), _VSTD::forward<_Args>(__args)...); }
+noexcept(noexcept(__bind_back_t<decay_t<_Fn>, tuple<decay_t<_Args>...>>(_VSTD::forward<_Fn>(__f), _VSTD::make_tuple(_VSTD::forward<_Args>(__args)...))))
+-> decltype(      __bind_back_t<decay_t<_Fn>, tuple<decay_t<_Args>...>>(_VSTD::forward<_Fn>(__f), _VSTD::make_tuple(_VSTD::forward<_Args>(__args)...)))
+{ return          __bind_back_t<decay_t<_Fn>, tuple<decay_t<_Args>...>>(_VSTD::forward<_Fn>(__f), _VSTD::make_tuple(_VSTD::forward<_Args>(__args)...)); }
 
 #endif // _LIBCPP_STD_VER > 17
 
diff --git a/libcxx/include/__utility/integer_sequence.h b/libcxx/include/__utility/integer_sequence.h
index 00d2fd51af6e..963c4a967070 100644
--- a/libcxx/include/__utility/integer_sequence.h
+++ b/libcxx/include/__utility/integer_sequence.h
@@ -74,39 +74,6 @@ template<size_t _Np>
 template<class... _Tp>
     using index_sequence_for = make_index_sequence<sizeof...(_Tp)>;
 
-
-template<class ..._Sequences>
-struct __join_integer_sequences_impl;
-
-template<class _S1>
-struct __join_integer_sequences_impl<_S1> {
-    using type = _S1;
-};
-
-template<class _Tp, _Tp ..._Ip1, _Tp ..._Ip2>
-struct __join_integer_sequences_impl<integer_sequence<_Tp, _Ip1...>, integer_sequence<_Tp, _Ip2...>> {
-    using type = integer_sequence<_Tp, _Ip1..., _Ip2...>;
-};
-
-template<class _S1, class _S2, class ..._Sn>
-struct __join_integer_sequences_impl<_S1, _S2, _Sn...>
-    : __join_integer_sequences_impl<typename __join_integer_sequences_impl<_S1, _S2>::type, _Sn...>
-{ };
-
-template<class ..._Sequences>
-using __join_integer_sequences = typename __join_integer_sequences_impl<_Sequences...>::type;
-
-template<size_t _Np, class _Sequence>
-struct __shift_index_sequence_by_impl;
-
-template<size_t _Np, size_t ..._Ips>
-struct __shift_index_sequence_by_impl<_Np, index_sequence<_Ips...>> {
-    using type = index_sequence<(_Np + _Ips)...>;
-};
-
-template<size_t _Np, class _Sequence>
-using __shift_index_sequence_by = typename __shift_index_sequence_by_impl<_Np, _Sequence>::type;
-
 #endif // _LIBCPP_STD_VER > 11
 
 _LIBCPP_END_NAMESPACE_STD