Task/Promise: Don't allow abstract types

Supporting abstract types only has little advantages and we might want
to store the result by value later.

3 files changed, 10 insertions, 16 deletions

diff --git a/src/base/QXmppPromise.h b/src/base/QXmppPromise.h
index 09bc5bcb..91443c7f 100644
--- a/src/base/QXmppPromise.h
+++ b/src/base/QXmppPromise.h
@@ -21,6 +21,7 @@
 template<typename T>
 class QXmppPromise
 {
+    static_assert(!std::is_abstract_v<T>);
 public:
     template<typename U = T, std::enable_if_t<std::is_void_v<U>> * = nullptr>
     QXmppPromise()
@@ -43,7 +44,7 @@ public:
 #ifdef QXMPP_DOC
     void reportFinished(T &&value)
 #else
-    template<typename U, std::enable_if_t<!std::is_void_v<T> && std::is_base_of_v<T, U>> * = nullptr>
+    template<typename U, std::enable_if_t<!std::is_void_v<T> && std::is_same_v<T, U>> * = nullptr>
     void finish(U &&value)
 #endif
     {
@@ -59,7 +60,7 @@ public:
     }
 
     /// \cond
-    template<typename U, std::enable_if_t<!std::is_void_v<T> && std::is_constructible_v<T, U> && !std::is_base_of_v<T, U>> * = nullptr>
+    template<typename U, std::enable_if_t<!std::is_void_v<T> && std::is_constructible_v<T, U> && !std::is_same_v<T, U>> * = nullptr>
     void finish(U &&value)
     {
         Q_ASSERT(!d.isFinished());
diff --git a/src/base/QXmppTask.h b/src/base/QXmppTask.h
index 141cf10e..84ba8a33 100644
--- a/src/base/QXmppTask.h
+++ b/src/base/QXmppTask.h
@@ -50,10 +50,6 @@ private:
 ///
 /// Tasks are generated by QXmppPromise and can be handled using QXmppTask::then().
 ///
-/// It supports abstract types, i.e. you can create a QXmppTask<QXmppStanza> and report MyIq
-/// (derived from QXmppStanza) to it and when handling the result it is possible to cast the result
-/// back to MyIq.
-///
 /// Unlike QFuture, this is not thread-safe. This avoids the need to do mutex locking at every
 /// access though.
 ///
@@ -177,10 +173,7 @@ public:
     ///
     /// Moves the result of the operation out of the task.
     ///
-    /// Since this returns by value, you will not be able to cast the result to derived types. Use
-    /// result() for read-only access or register a handler function using then().
-    ///
-    /// \warning This can only be used once the operation is finished.
+    /// \warning This can only be used once and only after the operation has finished.
     ///
 #ifdef QXMPP_DOC
     [[nodiscard]] T takeResult()
diff --git a/tests/qxmppclient/tst_qxmppclient.cpp b/tests/qxmppclient/tst_qxmppclient.cpp
index 0b5d33ea..a18fa0d2 100644
--- a/tests/qxmppclient/tst_qxmppclient.cpp
+++ b/tests/qxmppclient/tst_qxmppclient.cpp
@@ -174,8 +174,8 @@ void tst_QXmppClient::testTaskDirect()
     bool thenCalled = false;
     p.task().then(this, [&thenCalled](QXmppIq &&iq) {
         thenCalled = true;
-        QVERIFY(dynamic_cast<QXmppRegisterIq *>(&iq));
-        QCOMPARE(dynamic_cast<QXmppRegisterIq &>(iq).username(), QStringLiteral("username"));
+        // casting not supported
+        QVERIFY(!dynamic_cast<QXmppRegisterIq *>(&iq));
     });
     p.finish(std::move(iq));
 
@@ -203,8 +203,8 @@ void tst_QXmppClient::testTaskStore()
         thenCalled = true;
 
         QCOMPARE(iq.from(), QStringLiteral("juliet"));
-        QVERIFY(dynamic_cast<QXmppRegisterIq *>(&iq));
-        QCOMPARE(dynamic_cast<QXmppRegisterIq &>(iq).username(), QStringLiteral("username"));
+        // casting not supported
+        QVERIFY(!dynamic_cast<QXmppRegisterIq *>(&iq));
     });
     QVERIFY(thenCalled);
 
@@ -219,8 +219,8 @@ void tst_QXmppClient::testTaskStore()
     thenCalled = false;
     p.task().then(this, [&thenCalled](QXmppIq &&iq) {
         thenCalled = true;
-        QVERIFY(dynamic_cast<QXmppRegisterIq *>(&iq));
-        QCOMPARE(dynamic_cast<QXmppRegisterIq &>(iq).username(), QStringLiteral("username"));
+        // casting not supported
+        QVERIFY(!dynamic_cast<QXmppRegisterIq *>(&iq));
     });
     QVERIFY(thenCalled);