QVariant: prepare removal of nullptr_t special casing in Qt 7

customConstruct has to do an additional check to ensure that
QVariant::fromValue(nullptr) returns null.
We can get rid of it by special casing nullptr_t in a fromValue overload
instead, reducing the amount of work that needs to happen at runtime.

Change-Id: I2aea6aecfee0a9404cbd78dbea01a1d5d3047ca0
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/kernel/qvariant.cpp

@@ -250,7 +250,8 @@ static void customConstruct(const QtPrivate::QMetaTypeInterface *iface, QVariant
 
     // need to check for nullptr_t here, as this can get called by fromValue(nullptr). fromValue() uses
     // std::addressof(value) which in this case returns the address of the nullptr object.
-    d->is_null = !copy || isInterfaceFor<std::nullptr_t>(iface);
+    // ### Qt 7: remove nullptr_t special casing
+    d->is_null = !copy QT6_ONLY(|| isInterfaceFor<std::nullptr_t>(iface));
 
     void *dst;
     if (QVariant::Private::canUseInternalSpace(iface)) {

src/corelib/kernel/qvariant.h

@@ -435,6 +435,8 @@ public:
     static inline QVariant fromValue(const T &value)
 #endif
     {
+        if constexpr (std::is_null_pointer_v<T>)
+            return QVariant(QMetaType::fromType<std::nullptr_t>());
         return QVariant(QMetaType::fromType<T>(), std::addressof(value));
     }