[libcxx-commits] [libcxx] [libc++][ranges] Fix `ranges::join_view` segmented iterator trait (PR #158347)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-libcxx

Author: None (lbonn)

<details>
<summary>Changes</summary>

The outer iterator needs to move to the next segment when calling __compose.

Without this change, `find_segment_if` would never reach the end of the join_view which caused erroneous result when calling `ranges::find` on a join_view of bidirectional ranges.

Other specializations using the segmented iterator trait were likely to be affected as well.

Fixes #<!-- -->158279, #<!-- -->93180

cc @<!-- -->philnik777

---
Full diff: https://github.com/llvm/llvm-project/pull/158347.diff


2 Files Affected:

- (modified) libcxx/include/__ranges/join_view.h (+6-2) 
- (added) libcxx/test/std/ranges/range.adaptors/range.join/range.join.iterator/find.pass.cpp (+37) 


``````````diff
diff --git a/libcxx/include/__ranges/join_view.h b/libcxx/include/__ranges/join_view.h
index 327b349f476a7..6097754f403ec 100644
--- a/libcxx/include/__ranges/join_view.h
+++ b/libcxx/include/__ranges/join_view.h
@@ -410,8 +410,12 @@ struct __segmented_iterator_traits<_JoinViewIterator> {
 
   static constexpr _LIBCPP_HIDE_FROM_ABI _JoinViewIterator
   __compose(__segment_iterator __seg_iter, __local_iterator __local_iter) {
-    return _JoinViewIterator(
-        std::move(__seg_iter).__get_data(), std::move(__seg_iter).__get_iter(), std::move(__local_iter));
+    auto&& __outer = std::move(__seg_iter).__get_iter();
+    if (__local_iter == ranges::end(*__outer)) {
+      ++__outer;
+      return _JoinViewIterator(*std::move(__seg_iter).__get_data(), __outer);
+    }
+    return _JoinViewIterator(std::move(__seg_iter).__get_data(), __outer, std::move(__local_iter));
   }
 };
 
diff --git a/libcxx/test/std/ranges/range.adaptors/range.join/range.join.iterator/find.pass.cpp b/libcxx/test/std/ranges/range.adaptors/range.join/range.join.iterator/find.pass.cpp
new file mode 100644
index 0000000000000..f8bee2227d0b9
--- /dev/null
+++ b/libcxx/test/std/ranges/range.adaptors/range.join/range.join.iterator/find.pass.cpp
@@ -0,0 +1,37 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+
+#include <algorithm>
+#include <cassert>
+#include <ranges>
+#include <type_traits>
+
+#include "../types.h"
+
+constexpr bool test() {
+  // Test the segmented iterator implementation of join_view
+  // https://github.com/llvm/llvm-project/issues/158279
+  {
+    int buffer1[2][1] = {{1}, {2}};
+    auto joined       = std::views::join(buffer1);
+    assert(std::ranges::find(joined, 1) == std::ranges::begin(joined));
+    assert(std::ranges::find(joined, 2) == std::ranges::next(std::ranges::begin(joined)));
+    assert(std::ranges::find(joined, 3) == std::ranges::end(joined));
+  }
+
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+
+  return 0;
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/158347