Implement QPromise::tap(handler)

This `handler` allows to observe the value of the `input` promise, without changing the propagated value. The `output` promise will be resolved with the same value as the `input` promise (the `handler` returned value will be ignored). However, if `handler` throws, `output` is rejected with the new exception. Unlike `finally`, this handler is not called for rejections.

src/qtpromise/qpromise.h

@@ -43,6 +43,9 @@ public:
     template <typename THandler>
     inline QPromise<T> finally(THandler handler) const;
 
+    template <typename THandler>
+    inline QPromise<T> tap(THandler handler) const;
+
     inline QPromise<T> wait() const;
 
     void swap(QPromiseBase<T>& other) { qSwap(m_d, other.m_d); }

src/qtpromise/qpromise.inl

@@ -139,6 +139,16 @@ inline QPromise<T> QPromiseBase<T>::finally(THandler handler) const
     });
 }
 
+template <typename T>
+template <typename THandler>
+inline QPromise<T> QPromiseBase<T>::tap(THandler handler) const
+{
+    QPromise<T> p = *this;
+    return p.then(handler).then([=]() {
+        return p;
+    });
+}
+
 template <typename T>
 inline QPromise<T> QPromiseBase<T>::wait() const
 {