[libc++][ranges] Fix ranges::join_view segmented iterator trait

[lbonn]

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
Fixes #93180

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[llvmbot]

@llvm/pr-subscribers-libcxx

Author: None (lbonn)

Changes

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 --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;
+}

[lbonn]

I've moved the logic inside of the iterator itself instead of the segmented iterator trait which was meant to be kept simpler (I believe)

[lbonn]

Not sure what is going on with the macos failures, I don't think they are touching these changes.

   Failed Tests (9):
    llvm-libc++-shared.cfg.in :: std/re/re.alg/re.alg.match/awk.locale.pass.cpp
    llvm-libc++-shared.cfg.in :: std/re/re.alg/re.alg.match/basic.locale.pass.cpp
    llvm-libc++-shared.cfg.in :: std/re/re.alg/re.alg.match/ecma.locale.pass.cpp
    llvm-libc++-shared.cfg.in :: std/re/re.alg/re.alg.match/extended.locale.pass.cpp
    llvm-libc++-shared.cfg.in :: std/re/re.alg/re.alg.search/awk.locale.pass.cpp
    llvm-libc++-shared.cfg.in :: std/re/re.alg/re.alg.search/basic.locale.pass.cpp
    llvm-libc++-shared.cfg.in :: std/re/re.alg/re.alg.search/ecma.locale.pass.cpp
    llvm-libc++-shared.cfg.in :: std/re/re.alg/re.alg.search/extended.locale.pass.cpp
    llvm-libc++-shared.cfg.in :: std/re/re.traits/lookup_collatename.pass.cpp

They already have some XFAIL on some other linux and freebsd targets.

[philnik777]

Thanks! Do you need me to commit this?

[lbonn]

@philnik777 yes thank you, I would need your help for that.

In my opinion it is also worth a backport, I will request it through llvmbot once it is merged.

[frederick-vs-ja]

LGTM! Merging as CI failures are unrelated.

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[frederick-vs-ja]

/cherry-pick d1b5607

[llvmbot]

/pull-request #161008

[frederick-vs-ja]

@lbonn Could you make you mail address public for GitHub for next contribution(s)? We conventionally expect public mail address on GitHub.

⚠️ We detected that you are using a GitHub private e-mail address to contribute to the repo.
Please turn off Keep my email addresses private setting in your account.
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[lbonn]

@frederick-vs-ja oh no, sorry about that! I've made the change for next time.