QMetaType: Expand and clean up converter/view function documentation

This adds a few snippets to show how the functions can be used, and also
removes workarounds for oldver versions of qdoc that weren't clang based
- those workarounds also seem to have caused some bitrot.

Remove documentation for an overload of registerMutableView that
currently does not exist (for a member function with an ok parameter).

Fixes: QTBUG-96963
Change-Id: Ibc867d08bcacbc7add8c965df9864d6f339a5d54
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit 4f897584ec64d790ec8e58f10ad809847360d472)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

3 files changed, 118 insertions, 30 deletions

diff --git a/src/corelib/doc/snippets/qmetatype/registerConverters.cpp b/src/corelib/doc/snippets/qmetatype/registerConverters.cpp
new file mode 100644
index 0000000000..11e9ce1bfc
--- /dev/null
+++ b/src/corelib/doc/snippets/qmetatype/registerConverters.cpp
@@ -0,0 +1,101 @@
+/****************************************************************************
+**
+** Copyright (C) 2021 The Qt Company Ltd.
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of the documentation of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:BSD$
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** BSD License Usage
+** Alternatively, you may use this file under the terms of the BSD license
+** as follows:
+**
+** "Redistribution and use in source and binary forms, with or without
+** modification, are permitted provided that the following conditions are
+** met:
+**   * Redistributions of source code must retain the above copyright
+**     notice, this list of conditions and the following disclaimer.
+**   * Redistributions in binary form must reproduce the above copyright
+**     notice, this list of conditions and the following disclaimer in
+**     the documentation and/or other materials provided with the
+**     distribution.
+**   * Neither the name of The Qt Company Ltd nor the names of its
+**     contributors may be used to endorse or promote products derived
+**     from this software without specific prior written permission.
+**
+**
+** THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+** "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+** LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+** A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+** OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+** SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+** LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+** DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+** THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+** (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+** OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE."
+**
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+
+#include <QMetaType>
+#include <QString>
+
+int main() {
+  //! [member]
+  struct Coordinates {
+    int x;
+    int y;
+    int z;
+
+    QString toString() const { return u"[x: %1; y: %2, z: %3]"_qs.arg(QString::number(x),
+      QString::number(y),
+      QString::number(z)); }
+  };
+  QMetaType::registerConverter<Coordinates, QString>(&Coordinates::toString);
+  //! [member]
+
+  //! [memberOk]
+  struct BigNumber {
+      long long l;
+
+      int toInt(bool *ok = nullptr) const {
+        const bool canConvertSafely = l < std::numeric_limits<int>::max();
+        if (ok)
+          *ok = canConvertSafely;
+        return l;
+      }
+  };
+  QMetaType::registerConverter<BigNumber, int>(&BigNumber::toInt);
+  //! [memberOk]
+
+  //! [implicit]
+  class Counter {
+    int number = 0;
+  public:
+    int value() const { return number; }
+    operator int() const { return value(); }
+    void increment() {++number;}
+  };
+  QMetaType::registerConverter<Counter, int>();
+  //! [implicit]
+
+  struct CustomStringType {
+    const char *data() const {return nullptr;}
+  };
+  //! [unaryfunc]
+  QMetaType::registerConverter<CustomStringType, QString>([](const CustomStringType &str) {
+      return QString::fromUtf8(str.data());
+  });
+  //! [unaryfunc]
+}
diff --git a/src/corelib/kernel/qmetatype.cpp b/src/corelib/kernel/qmetatype.cpp
index 65745bf06a..641670022c 100644
--- a/src/corelib/kernel/qmetatype.cpp
+++ b/src/corelib/kernel/qmetatype.cpp
@@ -1598,30 +1598,42 @@ Q_GLOBAL_STATIC(QMetaTypeMutableViewRegistry, customTypesMutableViewRegistry)
     \since 5.2
     Registers the possibility of an implicit conversion from type From to type To in the meta
     type system. Returns \c true if the registration succeeded, otherwise false.
+
+    \snippet qmetatype/registerConverters.cpp [implicit]
 */
 
 /*!
-    \fn  template<typename MemberFunction, int> bool QMetaType::registerConverter(MemberFunction function)
+    \fn template<typename From, typename To> static bool registerConverter(To(From::*function)() const)
     \since 5.2
     \overload
     Registers a method \a function like To From::function() const as converter from type From
     to type To in the meta type system. Returns \c true if the registration succeeded, otherwise false.
+
+    \snippet qmetatype/registerConverters.cpp [member]
 */
 
 /*!
-    \fn template<typename MemberFunctionOk, char> bool QMetaType::registerConverter(MemberFunctionOk function)
+    \fn template<typename From, typename To> static bool registerConverter(To(From::*function)(bool*) const)
     \since 5.2
     \overload
     Registers a method \a function like To From::function(bool *ok) const as converter from type From
     to type To in the meta type system. Returns \c true if the registration succeeded, otherwise false.
+
+    The \a ok pointer can be used by the function to indicate whether the conversion succceeded.
+    \snippet qmetatype/registerConverters.cpp [memberOk]
+
 */
 
 /*!
-    \fn template<typename UnaryFunction> bool QMetaType::registerConverter(UnaryFunction function)
+    \fn template<typename From, typename To, typename UnaryFunction> static bool registerConverter(UnaryFunction function)
     \since 5.2
     \overload
     Registers a unary function object \a function as converter from type From
     to type To in the meta type system. Returns \c true if the registration succeeded, otherwise false.
+
+    \a function must take an instance of type \a From and return an instance of \a To. It can be a function
+    pointer, a lambda or a functor object.
+    \snippet qmetatype/registerConverters.cpp [unaryfunc]
 */
 
 /*!
@@ -1642,7 +1654,7 @@ bool QMetaType::registerConverterFunction(const ConverterFunction &f, QMetaType
 }
 
 /*!
-    \fn  template<typename MemberFunction, int> bool QMetaType::registerMutableView(MemberFunction function)
+    \fn template<typename From, typename To> static bool registerMutableView(To(From::*function)())
     \since 6.0
     \overload
     Registers a method \a function like \c {To From::function()} as mutable view of type \c {To} on
@@ -1651,16 +1663,7 @@ bool QMetaType::registerConverterFunction(const ConverterFunction &f, QMetaType
 */
 
 /*!
-    \fn template<typename MemberFunctionOk, char> bool QMetaType::registerMutableView(MemberFunctionOk function)
-    \since 6.0
-    \overload
-    Registers a method \a function like To From::function(bool *ok) as mutable view of type To on
-    type From in the meta type system. Returns \c true if the registration succeeded, otherwise
-    \c false.
-*/
-
-/*!
-    \fn template<typename UnaryFunction> bool QMetaType::registerMutableView(UnaryFunction function)
+    \fn template<typename From, typename To, typename UnaryFunction> static bool registerMutableView(UnaryFunction function)
     \since 6.0
     \overload
     Registers a unary function object \a function as mutable view of type To on type From
diff --git a/src/corelib/kernel/qmetatype.h b/src/corelib/kernel/qmetatype.h
index 1ca154a57d..35ab816042 100644
--- a/src/corelib/kernel/qmetatype.h
+++ b/src/corelib/kernel/qmetatype.h
@@ -541,21 +541,6 @@ public:
         return registerConverter<From, To>(QtPrivate::convertImplicit<From, To>);
     }
 
-#ifdef Q_CLANG_QDOC
-    template<typename MemberFunction, int>
-    static bool registerConverter(MemberFunction function);
-    template<typename MemberFunctionOk, char>
-    static bool registerConverter(MemberFunctionOk function);
-    template<typename UnaryFunction>
-    static bool registerConverter(UnaryFunction function);
-
-    template<typename MemberFunction, int>
-    static bool registerMutableView(MemberFunction function);
-    template<typename MemberFunctionOk, char>
-    static bool registerMutableView(MemberFunctionOk function);
-    template<typename UnaryFunction>
-    static bool registerMutableView(UnaryFunction function);
-#else
     // member function as in "QString QFont::toString() const"
     template<typename From, typename To>
     static bool registerConverter(To(From::*function)() const)
@@ -675,7 +660,6 @@ private:
             return false;
         }
     }
-#endif // Q_CLANG_DOC
 public:
 
     static bool convert(QMetaType fromType, const void *from, QMetaType toType, void *to);