QUtf8StringView: ensure binary compatibility throughout 6.x

We can't depend on whether the user compiles with -std=c++17 or
-std=c++20. So select what we can depend on and make that permanent.

Prior to this change:

 $ cat /tmp/test.cpp
 #include <QtCore/QUtf8StringView>
 void f(QUtf8StringView) {}
 $ qcd include
 $ g++ -S -o - -I. /tmp/test.cpp | grep globl | c++filt
         .globl  f(QBasicUtf8StringView<false>)
 $ g++ -fPIC -std=c++20 -S -o - -I. /tmp/test.cpp | grep globl | c++filt
         .globl  f(QBasicUtf8StringView<true>)

After this change, they're both "false". QUtf8StringView should have
been a concrete class that derived from QBsicUtf8StringView<whichever>
and inherited all its constructors. We'd cause ODR violations in C++20,
but nothing worse than what we usually do for BC reasons.

That solution is too late for Qt 6.x. Let's revisit in 7.0.

Change-Id: I6bcbe88c072a438b8b4efffd166e77199ecb39e3
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>
(cherry picked from commit c7deb3400faa1d9389137fe63ffbffade8203850)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

1 files changed, 1 insertions, 1 deletions

diff --git a/src/corelib/text/qstringalgorithms.h b/src/corelib/text/qstringalgorithms.h
index 5805e4c439..68d47b2893 100644
--- a/src/corelib/text/qstringalgorithms.h
+++ b/src/corelib/text/qstringalgorithms.h
@@ -48,7 +48,7 @@
 
 QT_BEGIN_NAMESPACE
 
-#ifdef __cpp_char8_t
+#if QT_VERSION >= QT_VERSION_CHECK(7, 0, 0)
 # define QT_BEGIN_HAS_CHAR8_T_NAMESPACE inline namespace q_has_char8_t {
 # define QT_BEGIN_NO_CHAR8_T_NAMESPACE namespace q_no_char8_t {
 #else