Add test with code from LWG-3698

Comment: #66033 (comment)
llvm · ldionne · Dec 12, 2023 · Sep 8, 2023 · Nov 27, 2023 · Nov 27, 2023
commit f256e1f9e663ec64ade659520b1b9494d8f0fe9d
@@ -0,0 +1,31 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+// UNSUPPORTED: no-localization
+
+// Check LWG-3698: `regex_iterator` and `join_view` don't work together very well
+
+#include <algorithm>
+#include <array>
+#include <cassert>
+#include <ranges>
+#include <regex>
+#include <string_view>
+
+int main(int, char**) {
+  char const text[] = "Hello";
+  std::regex regex{"[a-z]"};
+
+  auto lower =
+      std::ranges::subrange(
+          std::cregex_iterator(std::ranges::begin(text), std::ranges::end(text), regex), std::cregex_iterator{}) |
+      std::views::join | std::views::transform([](auto const& sm) { return std::string_view(sm.first, sm.second); });
+
+  assert(std::ranges::equal(lower, std::to_array<std::string_view>({"e", "l", "l", "o"})));
+}