Use std::shared_ptr for PromiseValue<T>::m_data instead of QSharedPoi…

…tner<T>

In optimized builds (such as -O2) GCC emits "maybe-uninitialized" warning for QSharedPointer<T>::d.  This happens only for optimized builds because that is when GCC tracks lifetimes[1]. In a project with lots of QtPromise uses this produces a heap of bogus warnings. This warning has been previously reported to a Qt team, but they're confident it is a compiler bug[2][3]. However, this is a second time and issue raised with QSharedPointer and warnings to it. The first time it was addressed here: simonbrunel#34.

[1] https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html#index-Wmaybe-uninitialized
[2] https://bugreports.qt.io/browse/QTBUG-14637
[3] https://bugreports.qt.io/browse/QTBUG-77641

src/qtpromise/qpromise_p.h

@@ -20,6 +20,8 @@
 #include <QtCore/QVariant>
 #include <QtCore/QVector>
 
+#include <memory>
+
 namespace QtPromise {
 
 template<typename T>
@@ -92,13 +94,13 @@ class PromiseValue
 {
 public:
     PromiseValue() { }
-    PromiseValue(const T& data) : m_data(QSharedPointer<T>::create(data)) { }
-    PromiseValue(T&& data) : m_data(QSharedPointer<T>::create(std::forward<T>(data))) { }
-    bool isNull() const { return m_data.isNull(); }
+    PromiseValue(const T& data) : m_data(std::make_shared<T>(data)) { }
+    PromiseValue(T&& data) : m_data(std::make_shared<T>(std::forward<T>(data))) { }
+    bool isNull() const { return m_data == nullptr; }
     const T& data() const { return *m_data; }
 
 private:
-    QSharedPointer<T> m_data;
+    std::shared_ptr<T> m_data;
 };
 
 class PromiseError