[clang] [clang-tools-extra] [libcxx] [llvm] [libc++][ranges] P2542R8: Implement `views::concat` (PR #120920)

[A. Jiang via cfe-commits]

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@@ -0,0 +1,70 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20, c++23
+
+#include <ranges>
+
+#include <cassert>
+#include <concepts>
+#include <initializer_list>
+#include <type_traits>
+#include <utility>
+#include <vector>
+
+#include "test_iterators.h"
+#include "test_range.h"
+
+struct Range : std::ranges::view_base {
+  using Iterator = forward_iterator<int*>;
+  using Sentinel = sentinel_wrapper<Iterator>;
+  constexpr explicit Range(int* b, int* e) : begin_(b), end_(e) {}
+  constexpr Iterator begin() const { return Iterator(begin_); }
+  constexpr Sentinel end() const { return Sentinel(Iterator(end_)); }
+
+private:
+  int* begin_;
+  int* end_;
+};
+
+template <typename View>
+constexpr void compareViews(View v, std::initializer_list<int> list) {
+  auto b1 = v.begin();
+  auto e1 = v.end();
+  auto b2 = list.begin();
+  auto e2 = list.end();
+  for (; b1 != e1 && b2 != e2;) {
+    assert(*b1 == *b2);
+    ++b1;
+    ++b2;
+  }
+  assert(b1 == e1);
+  assert(b2 == e2);
+}
+
+constexpr bool test() {
+  int buff[] = {0, 1, 2, 3, 4, 5, 6, 7};
+
+  {
+    Range range(buff, buff + 8);
+
+    {
+      decltype(auto) result = std::views::concat(range);
+      compareViews(result, {0, 1, 2, 3, 4, 5, 6, 7});
+    }
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frederick-vs-ja wrote:

I think we should test at least two cases, where `views::concat` returns `concat_view` for one but not another.

https://github.com/llvm/llvm-project/pull/120920