QStringView: add a test for overload resolution versus QString

In the presence of multiple overloads of a function taking either
QString or QStringView, QStringView should always be preferred.

The rationale is that the QStringView overload may have been
added "later" (read: the function was written when QStringView
was not available yet, so it took QString), and the fact that
a function with the _same name_ offers a QStringView overload
implies the function never needed to store/own the string in
the first place.

Add a (compile-time) test for this preference. This is in
preparation for a future QString(char16_t*) constructor
(in Qt 5.15 / Qt 6).

Change-Id: I60a435e494b653548f8f8d52c5d7e7cac2cc875a
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

1 files changed, 58 insertions, 0 deletions

diff --git a/tests/auto/corelib/text/qstringview/tst_qstringview.cpp b/tests/auto/corelib/text/qstringview/tst_qstringview.cpp
index 47ce9a6f63..631bcce508 100644
--- a/tests/auto/corelib/text/qstringview/tst_qstringview.cpp
+++ b/tests/auto/corelib/text/qstringview/tst_qstringview.cpp
@@ -221,6 +221,8 @@ private Q_SLOTS:
 
     void comparison();
 
+    void overloadResolution();
+
 private:
     template <typename String>
     void conversion_tests(String arg) const;
@@ -678,5 +680,61 @@ void tst_QStringView::comparison()
     QVERIFY(bb.compare(aa) > 0);
 }
 
+namespace QStringViewOverloadResolution {
+static void test(QString) = delete;
+static void test(QStringView) {}
+}
+
+// Compile-time only test: overload resolution prefers QStringView over QString
+void tst_QStringView::overloadResolution()
+{
+    {
+        QChar qcharArray[42] = {};
+        QStringViewOverloadResolution::test(qcharArray);
+        QChar *qcharPointer = qcharArray;
+        QStringViewOverloadResolution::test(qcharPointer);
+    }
+
+    {
+        ushort ushortArray[42] = {};
+        QStringViewOverloadResolution::test(ushortArray);
+        ushort *ushortPointer = ushortArray;
+        QStringViewOverloadResolution::test(ushortPointer);
+    }
+
+    {
+        QStringRef stringRef;
+        QStringViewOverloadResolution::test(stringRef);
+        QStringViewOverloadResolution::test(qAsConst(stringRef));
+        QStringViewOverloadResolution::test(std::move(stringRef));
+    }
+
+#if defined(Q_OS_WIN)
+    {
+        wchar_t wchartArray[42] = {};
+        QStringViewOverloadResolution::test(wchartArray);
+        QStringViewOverloadResolution::test(L"test");
+    }
+#endif
+
+#if defined(Q_COMPILER_UNICODE_STRINGS)
+    {
+        char16_t char16Array[] = u"test";
+        QStringViewOverloadResolution::test(char16Array);
+        char16_t *char16Pointer = char16Array;
+        QStringViewOverloadResolution::test(char16Pointer);
+    }
+#endif
+
+#if defined(Q_STDLIB_UNICODE_STRINGS)
+    {
+        std::u16string string;
+        QStringViewOverloadResolution::test(string);
+        QStringViewOverloadResolution::test(qAsConst(string));
+        QStringViewOverloadResolution::test(std::move(string));
+    }
+#endif
+}
+
 QTEST_APPLESS_MAIN(tst_QStringView)
 #include "tst_qstringview.moc"