From 287234704b57e171c786ba8df0b93c49edb903c5 Mon Sep 17 00:00:00 2001
From: Marc Mutz <marc.mutz@qt.io>
Date: Tue, 24 Dec 2024 13:34:25 +0100
Subject: [PATCH] QSpan: don't detach Qt containers

When converting an implicitly-shared Qt container to QSpan, the QSpan
ctor would call .data(), which detaches said Qt container (if it's not
const). This is what must happen for mutable spans (QSpan<int>), but
is not necessary for non-mutable spans (QSpan<const int>).

Fix by copying the potential const from QSpan::element_type to the
Range object in the resp. QSpan ctor. This makes a QSpan over a const
element_type mark the range const before passing it to adl_data() (=
member-data()). For a non-const element_type, nothing changes.

[ChangeLog][QtCore][QSpan] No longer detaches implicitly-shared Qt
containers converted to QSpan<const T, N>. Note that std::span<const
T, N> will, however, detach such containers, so we recommend to use
std::as_const() with implcitly-shared Qt containers, as always.

Fixes: QTBUG-132133
Pick-to: 6.9
Change-Id: I9fdae20994d2c900bc5b45b44db3901d10f8838a
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
---
 src/corelib/tools/qspan.h                    | 8 ++++++--
 src/corelib/tools/qspan.qdoc                 | 1 +
 tests/auto/corelib/tools/qspan/tst_qspan.cpp | 2 --
 3 files changed, 7 insertions(+), 4 deletions(-)

diff --git a/src/corelib/tools/qspan.h b/src/corelib/tools/qspan.h
index 6e4bf2dd16c..36a34b56edb 100644
--- a/src/corelib/tools/qspan.h
+++ b/src/corelib/tools/qspan.h
@@ -44,6 +44,10 @@ QT_END_INCLUDE_NAMESPACE
 
 namespace QSpanPrivate {
 
+template <typename From, typename To>
+std::conditional_t<std::is_const_v<From>, const To &, To &> // like [forward]/6.1 COPY_CONST
+const_propagated(To &in) { return in; }
+
 template <typename T, std::size_t E> class QSpanBase;
 
 template <typename T>
@@ -208,7 +212,7 @@ public:
 
     template <typename Range, typename Base::template if_compatible_range<Range> = true>
     Q_IMPLICIT constexpr QSpanBase(Range &&r)
-        : QSpanBase(QSpanPrivate::adl_data(r),  // no forward<>() here (std doesn't have it, either)
+        : QSpanBase(QSpanPrivate::adl_data(QSpanPrivate::const_propagated<T>(r)), // no forward<>() here (std doesn't have it, either)
                     qsizetype(QSpanPrivate::adl_size(r))) // ditto, no forward<>()
     {}
 
@@ -280,7 +284,7 @@ public:
 
     template <typename Range, typename Base::template if_compatible_range<Range> = true>
     Q_IMPLICIT constexpr QSpanBase(Range &&r)
-        : QSpanBase(QSpanPrivate::adl_data(r),  // no forward<>() here (std doesn't have it, either)
+        : QSpanBase(QSpanPrivate::adl_data(QSpanPrivate::const_propagated<T>(r)), // no forward<>() here (std doesn't have it, either)
                     qsizetype(QSpanPrivate::adl_size(r))) // ditto, no forward<>()
     {}
 
diff --git a/src/corelib/tools/qspan.qdoc b/src/corelib/tools/qspan.qdoc
index e7b5fe7dc0a..a2d93cbde68 100644
--- a/src/corelib/tools/qspan.qdoc
+++ b/src/corelib/tools/qspan.qdoc
@@ -104,6 +104,7 @@
     \list
     \li QSpan is using the signed qsizetype as \c{size_type}
         whereas \c{std::span} uses \c{size_t}.
+    \li (since Qt 6.9) \c{QSpan<const T>} doesn't detach Qt containers, \c{std::span} does.
     \li All QSpan constructors are implicit;
         many \c{std::span} ones are \c{explicit}.
     \li QSpan can be constructed from rvalue owning containers, \c{std::span} can not.
diff --git a/tests/auto/corelib/tools/qspan/tst_qspan.cpp b/tests/auto/corelib/tools/qspan/tst_qspan.cpp
index 2d63b0a45a2..fa2b81c8d34 100644
--- a/tests/auto/corelib/tools/qspan/tst_qspan.cpp
+++ b/tests/auto/corelib/tools/qspan/tst_qspan.cpp
@@ -478,7 +478,6 @@ void tst_QSpan::constQSpansDontDetachQtContainers() const
         [[maybe_unused]] const QList copy = li;
         QVERIFY(!li.isDetached());
         [[maybe_unused]] QSpan<const int> cvspan = li; // should not detach (QTBUG-132133)
-        QEXPECT_FAIL("", "QTBUG-132133", Continue);
         QVERIFY(!li.isDetached());
         [[maybe_unused]] QSpan<int> mvspan = li; // this _has_ to detach, though
         QVERIFY(li.isDetached());
@@ -489,7 +488,6 @@ void tst_QSpan::constQSpansDontDetachQtContainers() const
         [[maybe_unused]] const QList copy = li;
         QVERIFY(!li.isDetached());
         [[maybe_unused]] QSpan<const int, 4> cfspan = li; // should not detach (QTBUG-132133)
-        QEXPECT_FAIL("", "QTBUG-132133", Continue);
         QVERIFY(!li.isDetached());
         [[maybe_unused]] QSpan<int, 4> mfspan = li; // this _has_ to detach, though
         QVERIFY(li.isDetached());