Add a QVariant::compare() method

Add a method that allows comparing two variants. The method returns
a std::optional, as comparing two variants of different type is not
meaningful, or the types could not be comparable.

Change-Id: If4ae838d671e051dda1b474f25a2f9dcf85dc265
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/kernel/qvariant.cpp

@@ -2328,6 +2328,33 @@ bool QVariant::equals(const QVariant &v) const
     return metatype.equals(d.storage(), v.d.storage());
 }
 
+/*!
+    Compares the objects at \a lhs and \a rhs for ordering.
+
+    Returns an std::nullopt if comparison is not supported or the values are unordered.
+    Otherwise, returns -1, 0 or +1 according as \a lhs is less than, equal to or greater
+    than \a rhs.
+
+    If the variants contain data with a different metatype, the values are considered
+    unordered unless they are both of numeric or pointer types, where regular numeric or
+    pointer comparison rules will be used.
+
+    If both variants contain data of the same metatype, the method will use the
+    QMetaType::compare method to determine the ordering of the two variants, which can
+    also indicate that it can't establish an ordering between the two values.
+
+    \since 6.0
+    \sa QMetaType::compare(), QMetaType::isOrdered()
+*/
+std::optional<int> QVariant::compare(const QVariant &lhs, const QVariant &rhs)
+{
+    QMetaType t = lhs.d.type();
+    if (t != rhs.d.type())
+        return std::nullopt;
+    return t.compare(lhs.constData(), rhs.constData());
+}
+
+
 /*!
     \fn const void *QVariant::constData() const
     \fn const void* QVariant::data() const

src/corelib/kernel/qvariant.h

@@ -492,6 +492,8 @@ class Q_CORE_EXPORT QVariant
     inline bool operator!=(const QVariant &v) const
     { return !equals(v); }
 
+    static std::optional<int> compare(const QVariant &lhs, const QVariant &rhs);
+
 protected:
     friend inline bool operator==(const QVariant &, const QVariantComparisonHelper &);
 #ifndef QT_NO_DEBUG_STREAM