Properly detect and declare contiguous iterators

The recommended way to detect a contiguous iterator isn't to check
the iterator_category; it's to use the iterator concepts.

Similarly, the recommendation set in place by P2259 (for being
backwards-compatible) is to declare a iterator_concept member,
not to change iterator_category to a C++20 category, (also)
because legacy code may be checking for equality against a specific
category, rather than for convertibility. This is erroneous, but
such code exists, alas.

This is enshrined in C++20's stdlib: for instance, iterator_traits<Foo*>
has random_access_category_tag as iterator_category, but
contiguous_iterator_tag as its iterator_concept.

Hence:

1) in QArrayDataOps use the concept, and not the category, to do
the check

2) when declaring iterators, keep the category as random access,
and introduce the concept alias (if supported).

Pick-to: 6.2 6.3
Change-Id: Ib600da7331d687a15082becaa6be06aefc24bb9c
Reviewed-by: Andrei Golubev <andrei.golubev@qt.io>
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

2 files changed, 9 insertions, 16 deletions

diff --git a/src/corelib/tools/qarraydataops.h b/src/corelib/tools/qarraydataops.h
index 68a536004f..0de0c1df75 100644
--- a/src/corelib/tools/qarraydataops.h
+++ b/src/corelib/tools/qarraydataops.h
@@ -911,17 +911,13 @@ public:
         Q_ASSERT(distance >= 0 && distance <= this->allocatedCapacity() - this->size);
         Q_UNUSED(distance);
 
-#if __cplusplus >= 202002L
-        constexpr bool canUseCopyAppend = std::conjunction_v<
-                std::is_convertible<
-                    typename std::iterator_traits<It>::iterator_category,
-                    std::contiguous_iterator_tag
-                >,
-                std::is_same<
+#if __cplusplus >= 202002L && defined(__cpp_concepts) && defined(__cpp_lib_concepts)
+        constexpr bool canUseCopyAppend =
+                std::contiguous_iterator<It> &&
+                std::is_same_v<
                     std::remove_cv_t<typename std::iterator_traits<It>::value_type>,
                     T
-                >
-            >;
+                >;
         if constexpr (canUseCopyAppend) {
             this->copyAppend(std::to_address(b), std::to_address(e));
         } else
diff --git a/src/corelib/tools/qlist.h b/src/corelib/tools/qlist.h
index 8a2b542357..aaa2a950dd 100644
--- a/src/corelib/tools/qlist.h
+++ b/src/corelib/tools/qlist.h
@@ -147,12 +147,10 @@ public:
         using value_type = T;
         // libstdc++ shipped with gcc < 11 does not have a fix for defect LWG 3346
 #if __cplusplus >= 202002L && (!defined(_GLIBCXX_RELEASE) || _GLIBCXX_RELEASE >= 11)
-        using iterator_category = std::contiguous_iterator_tag;
+        using iterator_concept = std::contiguous_iterator_tag;
         using element_type = value_type;
-#else
-        using iterator_category = std::random_access_iterator_tag;
 #endif
-
+        using iterator_category = std::random_access_iterator_tag;
         using pointer = T *;
         using reference = T &;
 
@@ -220,11 +218,10 @@ public:
         using value_type = T;
         // libstdc++ shipped with gcc < 11 does not have a fix for defect LWG 3346
 #if __cplusplus >= 202002L && (!defined(_GLIBCXX_RELEASE) || _GLIBCXX_RELEASE >= 11)
-        using iterator_category = std::contiguous_iterator_tag;
+        using iterator_concept = std::contiguous_iterator_tag;
         using element_type = const value_type;
-#else
-        using iterator_category = std::random_access_iterator_tag;
 #endif
+        using iterator_category = std::random_access_iterator_tag;
         using pointer = const T *;
         using reference = const T &;