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>

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

src/corelib/text/qutf8stringview.h

@@ -356,6 +356,7 @@ private:
 template <bool UseChar8T>
 Q_DECLARE_TYPEINFO_BODY(QBasicUtf8StringView<UseChar8T>, Q_PRIMITIVE_TYPE);
 
+// ### Qt 7: remove the non-char8_t version of QUtf8StringView
 QT_BEGIN_NO_CHAR8_T_NAMESPACE
 using QUtf8StringView = QBasicUtf8StringView<false>;
 QT_END_NO_CHAR8_T_NAMESPACE

src/corelib/text/qutf8stringview.qdoc

@@ -116,42 +116,14 @@
     The second, in namespace \c{q_has_char8_t}, has a value_type of
     \c{const char8_t} and is only available when compiling in C++20 mode.
 
-    In C++17 mode, \c{q_no_char8_t} is an inline namespace, in C++20 it's
-    \c{q_has_char8_t}. This means that the name "QUtf8StringView" (without
-    explicit namespace) will denote different types in C++17 and C++20 modes.
+    \c{q_no_char8_t} is an inline namespace regardless of C++ edition, to avoid
+    accidental binary incompatibilities. To use the \c{char8_t} version, you
+    need to name it explicitly with \c{q_has_char8_t::QUtf8StringView}.
 
     Internally, both are instantiations of the same template class,
     QBasicUtf8StringView. Please do not use the template class's name in your
     source code.
 
-    All Qt APIs use \c{q_no_char8_t::QUtf8StringView} due to binary compatibility,
-    but these APIs accept \c{q_has_char8_t::QUtf8StringView} as well, since the
-    latter implicitly converts into the former, and vice versa.
-
-    In your own code, please use only \c QUtf8StringView and/or
-    \c{q_no_char8_t::QUtf8StringView}:
-
-    \list
-    \li If you only target C++20, then use "QUtf8StringView". It will be an alias
-        for \c{q_has_char8_t::QUtf8StringView} and you'll never look back.
-    \li If you only target C++17, then use "QUtf8StringView". It will be an alias
-        for \c{q_no_char8_t::QUtf8StringView} and for the time being, you're ok.
-    \li If you target both C++17 and C++20, then you have a choice to make:
-        \list
-        \li If you don't mind the source-incompatibility of return values of
-            QUtf8StringView::data() etc changing when compiling under C++17 or C++20,
-            use "QUtf8StringView". You will need to write your code in such a way that
-            it adapts to the differences in the QUtf8StringView API in different C++
-            versions.
-        \li If you don't want to deal with the above source-incompatibilities, or if
-            you need to maintain binary compatibility between C++20 and C++17 builds,
-            use "q_no_char8_t::QUtf8StringView" explicitly. Be aware that the
-            \c{q_no_char8_t} version will disappear in Qt 7.
-        \endlist
-    \endlist
-
-    Taken together: Just use QUtf8StringView unless you know what you're doing.
-
     \sa QAnyStringView, QUtf8StringView, QString
 */