[gcc r16-6764] libstdc++: Add comment justifying separate proxy_random_access_iterator_wrapper.

[Tomasz Kaminski]

https://gcc.gnu.org/g:e16de4a10f5874f83a53dd7216c7919375b34d94

commit r16-6764-ge16de4a10f5874f83a53dd7216c7919375b34d94
Author: Tomasz Kamiński <tkaminsk@redhat.com>
Date:   Wed Jan 14 10:35:59 2026 +0100

    libstdc++: Add comment justifying separate proxy_random_access_iterator_wrapper.
    
    It meets Cpp17RandomAccessIterator requirements, but does not satisfy
    random_access_iterator concept.
    
    libstdc++-v3/ChangeLog:
    
            * testsuite/util/testsuite_iterators.h: Modify comment.
    
    Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
    Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>

Diff:
---
 libstdc++-v3/testsuite/util/testsuite_iterators.h | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/libstdc++-v3/testsuite/util/testsuite_iterators.h b/libstdc++-v3/testsuite/util/testsuite_iterators.h
index caede49e41b5..ca022e5d0f9c 100644
--- a/libstdc++-v3/testsuite/util/testsuite_iterators.h
+++ b/libstdc++-v3/testsuite/util/testsuite_iterators.h
@@ -695,9 +695,12 @@ namespace __gnu_test
    * @brief random_access_iterator wrapper for pointer,
    * that returns proxy from subscript.
    *
-   * This class takes a pointer and wraps it to provide exactly
-   * the requirements of a random_access_iterator. It should not be
-   * instantiated directly, but generated from a test_container
+   * This is separate from random_access_iterator_wrapper (that returns
+   * T& from operator subscript), as it meets the requirements of
+   * Cpp17RandomAccessIterator (C++20 [tab:randomaccessiterator])
+   * that allows to return type that is convertible to reference,
+   * but does not satisfy random_access_iterator concept
+   * (C++20 [random_access_iterator]).
    */
   template<class T>
   struct proxy_random_access_iterator_wrapper