[libcxx-commits] [libcxx] [libc++] Introduce `__product_iterator_traits` and optimise `flat_map::insert` (PR #139454)

[A. Jiang via libcxx-commits]

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+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___ITERATOR_PRODUCT_ITERATOR_H
+#define _LIBCPP___ITERATOR_PRODUCT_ITERATOR_H
+
+// Product iterators are iterators that contain two or more underlying iterators.
+//
+// For example, std::flat_map stores its data into two separate containers, and its iterator
+// is a proxy over two separate underlying iterators. The concept of product iterators
+// allows algorithms to operate over these underlying iterators separately, opening the
+// door to various optimizations.
+//
+// If __product_iterator_traits can be instantiated, the following functions and associated types must be provided:
+// - static constexpr size_t Traits::__size
+//   The number of underlying iterators inside the product iterator.
+//
+// - template <size_t _N>
+//   static decltype(auto) Traits::__get_iterator_element(It&& __it)
+//   Returns the _Nth iterator element of the given product iterator.
+
+#include <__config>
+#include <__cstddef/size_t.h>
+#include <__type_traits/enable_if.h>
+#include <__type_traits/integral_constant.h>
+#include <__utility/declval.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+template <class _Iterator>
+struct __product_iterator_traits;
+/* exposition-only:
+{
+  static constexpr size_t __size = ...;
+
+  template <size_t _N, class _Iter>
+  static decltype(auto) __get_iterator_element(_Iter&&);
+
+  template <class... _Iters>
+  static _Iterator __make_product_iterator(_Iters&&...);
+};
+*/
+
+template <class _Tp, size_t = 0>
+struct __is_product_iterator : false_type {};
+
+template <class _Tp>
+struct __is_product_iterator<_Tp, sizeof(__product_iterator_traits<_Tp>) * 0> : true_type {};
+
+template <class _Tp, size_t _Size, class = void>
+struct __is_product_iterator_of_size : false_type {};
+
+template <class _Tp, size_t _Size>
+struct __is_product_iterator_of_size<_Tp, _Size, __enable_if_t<__product_iterator_traits<_Tp>::__size == _Size> >
+    : true_type {};
+
+template <class _Iterator, size_t _Nth>
+using __product_iterator_element_t _LIBCPP_NODEBUG =
+    decltype(__product_iterator_traits<_Iterator>::__get_iterator_element<_Nth>(std::declval<_Iterator>()));
----------------
frederick-vs-ja wrote:

Looks like we need to add `template` here.
```suggestion
    decltype(__product_iterator_traits<_Iterator>::template __get_iterator_element<_Nth>(std::declval<_Iterator>()));
```
(Although I wonder whether there should be a paper making `template` optional here.)

https://github.com/llvm/llvm-project/pull/139454