Add a helper for better error messages when functor is incompatible

Amends 207aae5560aa2865ec55ddb9ecbb50048060c0c0 to make it easy to
create human-friendly error messages. Since the functor-accepting member
functions are not removed from the API, the first compile error will be
that there is no suitable overload of the makeSlotObject helper, which.
With the assert helper, the first error message is easier to understand.

Change-Id: I4878ec35a44ddfa5dc9d9e358d81c3fd40389c0c
Reviewed-by: Qt CI Bot <[email protected]>
Reviewed-by: Thiago Macieira <[email protected]>

4 files changed, 19 insertions, 8 deletions

diff --git a/src/corelib/kernel/qcoreapplication.h b/src/corelib/kernel/qcoreapplication.h
index 2514c7630b7..ae01cdee9ae 100644
--- a/src/corelib/kernel/qcoreapplication.h
+++ b/src/corelib/kernel/qcoreapplication.h
@@ -126,6 +126,7 @@ public:
                            Functor &&func)
     {
         using Prototype = void(*)(QPermission);
+        QtPrivate::AssertCompatibleFunctions<Prototype, Functor>();
         requestPermission(permission,
                           QtPrivate::makeSlotObject<Prototype>(std::forward<Functor>(func)),
                           receiver);
diff --git a/src/corelib/kernel/qobjectdefs_impl.h b/src/corelib/kernel/qobjectdefs_impl.h
index e8b9b4a2091..721193fe914 100644
--- a/src/corelib/kernel/qobjectdefs_impl.h
+++ b/src/corelib/kernel/qobjectdefs_impl.h
@@ -512,6 +512,21 @@ namespace QtPrivate {
                                                      ActualArguments, void>(std::forward<Functor>(func));
         }
     }
+
+    template<typename Prototype, typename Functor, typename = void>
+    struct AreFunctionsCompatible : std::false_type {};
+    template<typename Prototype, typename Functor>
+    struct AreFunctionsCompatible<Prototype, Functor, std::enable_if_t<
+        std::is_same_v<decltype(QtPrivate::makeSlotObject<Prototype>(std::forward<Functor>(std::declval<Functor>()))),
+        QtPrivate::QSlotObjectBase *>>
+    > : std::true_type {};
+
+    template<typename Prototype, typename Functor>
+    inline constexpr bool AssertCompatibleFunctions() {
+        static_assert(AreFunctionsCompatible<Prototype, Functor>::value,
+                      "Functor is not compatible with expected prototype!");
+        return true;
+    }
 }
 
 QT_END_NAMESPACE
diff --git a/src/network/kernel/qhostinfo.h b/src/network/kernel/qhostinfo.h
index 9b420bd3ca9..1f18bf83027 100644
--- a/src/network/kernel/qhostinfo.h
+++ b/src/network/kernel/qhostinfo.h
@@ -66,6 +66,7 @@ public:
                                  Functor &&func)
     {
         using Prototype = void(*)(QHostInfo);
+        QtPrivate::AssertCompatibleFunctions<Prototype, Functor>();
         return lookupHostImpl(name, receiver,
                               QtPrivate::makeSlotObject<Prototype>(std::forward<Functor>(func)),
                               nullptr);
diff --git a/tests/auto/corelib/kernel/qobject/tst_qobject.cpp b/tests/auto/corelib/kernel/qobject/tst_qobject.cpp
index 649f075a02e..76ceb842715 100644
--- a/tests/auto/corelib/kernel/qobject/tst_qobject.cpp
+++ b/tests/auto/corelib/kernel/qobject/tst_qobject.cpp
@@ -8377,6 +8377,7 @@ public:
     template<typename Functor>
     bool callMe0(const typename QtPrivate::ContextTypeForFunctor<Functor>::ContextType *, Functor &&func)
     {
+        QtPrivate::AssertCompatibleFunctions<Prototype0, Functor>();
         auto *slotObject = QtPrivate::makeSlotObject<Prototype0>(std::forward<Functor>(func));
         slotObject->destroyIfLastRef();
         return true;
@@ -8391,6 +8392,7 @@ public:
     template<typename Functor>
     bool callMe1(const typename QtPrivate::ContextTypeForFunctor<Functor>::ContextType *, Functor &&func)
     {
+        QtPrivate::AssertCompatibleFunctions<Prototype1, Functor>();
         auto *slotObject = QtPrivate::makeSlotObject<Prototype1>(std::forward<Functor>(func));
         slotObject->destroyIfLastRef();
         return true;
@@ -8406,14 +8408,6 @@ public:
 static void freeFunction0() {}
 static void freeFunction1(QString) {}
 
-template<typename Prototype, typename Functor, typename = void>
-struct AreFunctionsCompatible : std::false_type {};
-template<typename Prototype, typename Functor>
-struct AreFunctionsCompatible<Prototype, Functor, std::enable_if_t<
-    std::is_same_v<decltype(QtPrivate::makeSlotObject<Prototype>(std::forward<Functor>(std::declval<Functor>()))),
-    QtPrivate::QSlotObjectBase *>>
-> : std::true_type {};
-
 template<typename Prototype, typename Functor>
 inline constexpr bool compiles(Functor &&) {
     return QtPrivate::AreFunctionsCompatible<Prototype, Functor>::value;