Implement QtPromise::attempt(functor, args...)

Add a new helper that calls functor immediately and returns a promise fulfilled with the value returned by functor. Any synchronous exceptions will be turned into rejections on the returned promise. This is a convenient method that can be used instead of handling both synchronous and asynchronous exception flows.

Also simplify PromiseDispatch which now calls the functor with a variable number of arguments (including none).

tests/auto/qtpromise/helpers/attempt/attempt.pro

@@ -0,0 +1,5 @@
+QT += concurrent
+TARGET = tst_helpers_attempt
+SOURCES += $$PWD/tst_attempt.cpp
+
+include(../../qtpromise.pri)

tests/auto/qtpromise/helpers/attempt/tst_attempt.cpp

@@ -0,0 +1,99 @@
+// Tests
+#include "../../shared/utils.h"
+
+// QtPromise
+#include <QtPromise>
+
+// Qt
+#include <QtConcurrent>
+#include <QtTest>
+
+// STL
+#include <memory>
+
+using namespace QtPromise;
+
+class tst_helpers_attempt : public QObject
+{
+    Q_OBJECT
+
+private Q_SLOTS:
+    void voidResult();
+    void typedResult();
+    void futureResult();
+    void promiseResult();
+    void functorThrows();
+    void callWithParams();
+};
+
+QTEST_MAIN(tst_helpers_attempt)
+#include "tst_attempt.moc"
+
+void tst_helpers_attempt::voidResult()
+{
+    auto p = QtPromise::attempt([]() {});
+
+    Q_STATIC_ASSERT((std::is_same<decltype(p), QPromise<void>>::value));
+    QCOMPARE(p.isFulfilled(), true);
+    QCOMPARE(waitForValue(p, -1, 42), 42);
+}
+
+void tst_helpers_attempt::typedResult()
+{
+    auto p = QtPromise::attempt([]() {
+        return QString("foo");
+    });
+
+    Q_STATIC_ASSERT((std::is_same<decltype(p), QPromise<QString>>::value));
+    QCOMPARE(p.isFulfilled(), true);
+    QCOMPARE(waitForValue(p, QString()), QString("foo"));
+}
+
+void tst_helpers_attempt::futureResult()
+{
+    auto p = QtPromise::attempt([]() {
+        return QtConcurrent::run([]() {
+            return QString("foo");
+        });
+    });
+
+    Q_STATIC_ASSERT((std::is_same<decltype(p), QPromise<QString>>::value));
+    QCOMPARE(p.isPending(), true);
+    QCOMPARE(waitForValue(p, QString()), QString("foo"));
+}
+
+void tst_helpers_attempt::promiseResult()
+{
+    auto p = QtPromise::attempt([]() {
+        return QtPromise::qPromise(42).delay(200);
+    });
+
+    Q_STATIC_ASSERT((std::is_same<decltype(p), QPromise<int>>::value));
+    QCOMPARE(p.isPending(), true);
+    QCOMPARE(waitForValue(p, -1), 42);
+}
+
+void tst_helpers_attempt::functorThrows()
+{
+    auto p = QtPromise::attempt([]() {
+        if (true) {
+            throw QString("bar");
+        }
+        return 42;
+    });
+
+    Q_STATIC_ASSERT((std::is_same<decltype(p), QPromise<int>>::value));
+    QCOMPARE(p.isRejected(), true);
+    QCOMPARE(waitForError(p, QString()), QString("bar"));
+}
+
+void tst_helpers_attempt::callWithParams()
+{
+    auto p = QtPromise::attempt([&](int i, const QString& s) {
+        return QString("%1:%2").arg(i).arg(s);
+    }, 42, "foo");
+
+    Q_STATIC_ASSERT((std::is_same<decltype(p), QPromise<QString>>::value));
+    QCOMPARE(p.isFulfilled(), true);
+    QCOMPARE(waitForValue(p, QString()), QString("42:foo"));
+}

tests/auto/qtpromise/helpers/helpers.pro

@@ -1,6 +1,7 @@
 TEMPLATE = subdirs
 SUBDIRS += \
     all \
+    attempt \
     filter \
     map \
     reject \