QByteArray: add conversion to std::string_view

96d67da420697cee10bdc537a1a592f6f22e2b8f didn't originally add this
conversion because on certain compilers (QNX) it's ambiguous. The
problem seems to be that QByteArray by default is convertible to const
char *; by adding a conversion to std::string_view, then the explicit
conversion

  std::string_view v(ba);

somehow becomes ambiguous.

Still, we've decided that we want the implicit conversion on non-broken
compilers, so this commits add it there.

[ChangeLog][QtCore][QByteArray] Added an implicit conversion operator
towards std::string_view.

[ChangeLog][QtCore][Important Behavior Changes] Converting a QByteArray
to a std::string_view will now create a view span over the entirety of
the byte array. Before, the view would have stopped at the first NUL in
the byte array (if there was one), unless the QT_NO_CAST_FROM_BYTEARRAY
macro was defined.

Change-Id: I70bbddac97636fbedb4d449eaa65cad4ecbdf0bf
Reviewed-by: Ahmad Samir <a.samirh78@gmail.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/text/qbytearray.cpp

@@ -4837,6 +4837,15 @@ std::string QByteArray::toStdString() const
     return std::string(data(), size_t(size()));
 }
 
+/*!
+    \fn QByteArray::operator std::string_view() const noexcept
+    \since 6.10
+
+    Converts this QByteArray object to a \c{std::string_view} object.
+    The returned string view will span over the entirety of the byte
+    array.
+*/
+
 /*!
     \since 4.4
 

src/corelib/text/qbytearray.h

@@ -118,6 +118,18 @@ public:
     inline operator const char *() const;
     inline operator const void *() const;
 #endif
+
+    // Some compilers consider this conversion ambiguous, so
+    // we're not offering it there:
+    // * QCC 8.3 on QNX
+    // * GHS 2022.1.4 on INTEGRITY
+#if (!defined(Q_OS_QNX) || !defined(Q_CC_GNU_ONLY) || Q_CC_GNU_ONLY > 803) && \
+    (!defined(Q_CC_GHS) || !defined(__GHS_VERSION_NUMBER) || __GHS_VERSION_NUMBER > 202214)
+# define QT_BYTEARRAY_CONVERTS_TO_STD_STRING_VIEW
+    Q_IMPLICIT operator std::string_view() const noexcept
+    { return std::string_view(data(), std::size_t(size())); }
+#endif
+
     inline char *data();
     inline const char *data() const noexcept;
     const char *constData() const noexcept { return data(); }

tests/auto/corelib/text/qbytearray/tst_qbytearray.cpp

@@ -148,6 +148,7 @@ private slots:
     void length();
     void length_data();
     void slice() const;
+    void std_stringview_conversion();
 };
 
 static const QByteArray::DataPointer staticStandard = {
@@ -3202,5 +3203,38 @@ void tst_QByteArray::slice() const
     QVERIFY(a.isEmpty());
 }
 
+void tst_QByteArray::std_stringview_conversion()
+{
+#ifdef QT_BYTEARRAY_CONVERTS_TO_STD_STRING_VIEW
+    static_assert(std::is_convertible_v<QByteArray, std::string_view>);
+
+    QByteArray ba;
+    std::string_view sv = ba;
+    QCOMPARE(sv, std::string_view());
+    sv = std::string_view(ba);
+    QCOMPARE(sv, std::string_view());
+
+    ba = ""_ba;
+    sv = ba;
+    QCOMPARE(ba.size(), 0);
+    QCOMPARE(sv.size(), size_t(0));
+    QCOMPARE(sv, std::string_view());
+
+    ba = "Hello"_ba;
+    sv = ba;
+    QCOMPARE(ba.size(), 5);
+    QCOMPARE(sv.size(), size_t(5));
+    QCOMPARE(sv, std::string_view("Hello"));
+
+    ba = "Hello\0world"_ba;
+    sv = ba;
+    QCOMPARE(ba.size(), 11);
+    QCOMPARE(sv.size(), size_t(11));
+    QCOMPARE(sv, std::string_view("Hello\0world", 11));
+#else
+    QSKIP("This compiler does not support QByteArray -> std::string_view implicit conversions");
+#endif
+}
+
 QTEST_MAIN(tst_QByteArray)
 #include "tst_qbytearray.moc"