QString: add char8_t overload of fromUtf8()

Use the overload-with-template trick from P1423 to avoid ambiguities
when existing callers pass 0 or nullptr.

Add a qdoc-ignored macro to hide the fact that the overload is a
template.

[ChangeLog][QtCore][QString] Added char8_t overload of fromUtf8().

Change-Id: Iaa2d365bfa161ef36cc73fa3bad50aabf34d01db
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

6 files changed, 28 insertions, 1 deletions

diff --git a/doc/global/qt-cpp-defines.qdocconf b/doc/global/qt-cpp-defines.qdocconf
index d3c516a656..fd5eb46926 100644
--- a/doc/global/qt-cpp-defines.qdocconf
+++ b/doc/global/qt-cpp-defines.qdocconf
@@ -132,6 +132,7 @@ Cpp.ignoretokens        += \
                           Q_SVG_EXPORT \
                           Q_TESTLIB_EXPORT \
                           Q_TYPENAME \
+                          Q_WEAK_OVERLOAD \
                           Q_WIDGETS_EXPORT \
                           Q_WINEXTRAS_EXPORT \
                           Q_XML_EXPORT \
diff --git a/src/corelib/global/qcompilerdetection.h b/src/corelib/global/qcompilerdetection.h
index 7b86279670..6341d060f1 100644
--- a/src/corelib/global/qcompilerdetection.h
+++ b/src/corelib/global/qcompilerdetection.h
@@ -1233,6 +1233,12 @@
 #endif
 
 /*
+ * "Weak overloads" - makes an otherwise confliciting overload weaker
+ * (by making it a template)
+ */
+#define Q_WEAK_OVERLOAD template <typename = void>
+
+/*
  * Warning/diagnostic handling
  */
 
diff --git a/src/corelib/text/qstring.cpp b/src/corelib/text/qstring.cpp
index 16e9f1d63b..dff0dbce7f 100644
--- a/src/corelib/text/qstring.cpp
+++ b/src/corelib/text/qstring.cpp
@@ -5098,6 +5098,14 @@ QString QString::fromLocal8Bit_helper(const char *str, int size)
 */
 
 /*!
+    \fn QString QString::fromUtf8(const char8_t *str, qsizetype size)
+    \overload
+    \since 6.0
+
+    This overload is only available when compiling in C++20 mode.
+*/
+
+/*!
     \fn QString QString::fromUtf8(const QByteArray &str)
     \overload
     \since 5.0
diff --git a/src/corelib/text/qstring.h b/src/corelib/text/qstring.h
index f14644d956..b77c6134d5 100644
--- a/src/corelib/text/qstring.h
+++ b/src/corelib/text/qstring.h
@@ -658,6 +658,11 @@ public:
     {
         return fromUtf8_helper(str, (str && size == -1) ? int(strlen(str)) : size);
     }
+#ifdef __cpp_char8_t
+    Q_WEAK_OVERLOAD
+    static inline QString fromUtf8(const char8_t *str, qsizetype size = -1)
+    { return fromUtf8(reinterpret_cast<const char *>(str), int(size)); }
+#endif
     static inline QString fromLocal8Bit(const char *str, int size = -1)
     {
         return fromLocal8Bit_helper(str, (str && size == -1) ? int(strlen(str)) : size);
diff --git a/tests/auto/corelib/text/qstring/qstring.pro b/tests/auto/corelib/text/qstring/qstring.pro
index 5fff5530b7..9a478b134e 100644
--- a/tests/auto/corelib/text/qstring/qstring.pro
+++ b/tests/auto/corelib/text/qstring/qstring.pro
@@ -3,7 +3,8 @@ TARGET = tst_qstring
 QT = core-private testlib
 SOURCES = tst_qstring.cpp
 # DEFINES += QT_NO_CAST_TO_ASCII # actively #undef-ed by tst_qstring.cpp
-qtConfig(c++11): CONFIG += c++11
+qtConfig(c++1z): CONFIG += c++1z
+qtConfig(c++2a): CONFIG += c++2a
 DEFINES += QT_DISABLE_DEPRECATED_BEFORE=0
 
 !qtConfig(doubleconversion):!qtConfig(system-doubleconversion) {
diff --git a/tests/auto/corelib/text/qstring/tst_qstring.cpp b/tests/auto/corelib/text/qstring/tst_qstring.cpp
index 5c46ae6ae1..61f775f4d8 100644
--- a/tests/auto/corelib/text/qstring/tst_qstring.cpp
+++ b/tests/auto/corelib/text/qstring/tst_qstring.cpp
@@ -4216,9 +4216,15 @@ void tst_QString::nullFromUtf8()
     a = QString::fromUtf8(0);
     QVERIFY(a.isNull());
     QVERIFY(a.isEmpty());
+    a = QString::fromUtf8(nullptr);
+    QVERIFY(a.isNull());
+    QVERIFY(a.isEmpty());
     a = QString::fromUtf8("");
     QVERIFY(!a.isNull());
     QVERIFY(a.isEmpty());
+    a = QString::fromUtf8(u8""); // char in C++17 / char8_t in C++20
+    QVERIFY(!a.isNull());
+    QVERIFY(a.isEmpty());
     a = QString::fromUtf8(QByteArray());
     QVERIFY(a.isNull());
     QVERIFY(a.isEmpty());