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

[Louis Dionne 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:
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ldionne wrote:

Missing documentation for `__make_product_iterator`!

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