Improve error reporting when requesting unsupported native interface

By switching out the static_assert for an enable_if we end up producing
a clearer error, at the call site:

/qt/qtbase/examples/gui/rasterwindow/main.cpp:69:9: error: no matching member
      function for call to 'nativeInterface'
    app.nativeInterface<QNativeInterface::QCocoaGLContext>();
    ~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/qt/qtbase/src/gui/kernel/qguiapplication.h:176:5: note:
      candidate template ignored: requirement
      'NativeInterface<QNativeInterface::QCocoaGLContext>::isCompatibleWith<QGuiApplication>'
      was not satisfied [with NativeInterface = QNativeInterface::QCocoaGLContext, TypeInfo =
      QNativeInterface::Private::NativeInterface<QNativeInterface::QCocoaGLContext>, BaseType =
      QGuiApplication]
    QT_DECLARE_NATIVE_INTERFACE_ACCESSOR(QGuiApplication)
    ^
By using SFINAE for the TypeInfo we can also ensure that it works for
types that are not native interfaces, such as if the user tries to
call nativeInterface<QString>().

Since we can no longer use decltype(*this) to resolve the base type
we need to change QT_DECLARE_NATIVE_INTERFACE_ACCESSOR to take the
type as an argument, as we do for other QT_DECLARE_FOO macros.

Pick-to: 6.2
Change-Id: Ie3f7e01ab7c3eb3dcc2ef730834f268bb9e81e0c
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

1 files changed, 91 insertions, 17 deletions

diff --git a/src/corelib/global/qnativeinterface.h b/src/corelib/global/qnativeinterface.h
index d32e2cd982..395a4ba1da 100644
--- a/src/corelib/global/qnativeinterface.h
+++ b/src/corelib/global/qnativeinterface.h
@@ -61,11 +61,21 @@ QT_BEGIN_NAMESPACE
 #define QT_DECLARE_NATIVE_INTERFACE_3(NativeInterface, Revision, BaseType) \
     protected: \
         virtual ~NativeInterface(); \
+        \
         struct TypeInfo { \
             using baseType = BaseType; \
             static constexpr char const *name = QT_STRINGIFY(NativeInterface); \
             static constexpr int revision = Revision; \
         }; \
+        \
+        template <typename, typename> \
+        friend struct QNativeInterface::Private::has_type_info; \
+        \
+        template <typename> \
+        friend bool constexpr QNativeInterface::Private::hasTypeInfo(); \
+        \
+        template <typename> \
+        friend struct QNativeInterface::Private::TypeInfo; \
     public: \
 
 // Revisioned interfaces only make sense when exposed through a base
@@ -81,35 +91,97 @@ QT_BEGIN_NAMESPACE
     QT_OVERLOADED_MACRO(QT_DECLARE_NATIVE_INTERFACE, __VA_ARGS__)
 
 namespace QNativeInterface::Private {
+
+    // Basic type-trait to verify that a given native interface has
+    // all the required type information for us to evaluate it.
+    template <typename NativeInterface, typename = void>
+    struct has_type_info : std::false_type {};
+
+    // The type-trait is friended by TypeInfo, so that we can
+    // evaluate TypeInfo in the template arguments.
+    template <typename NativeInterface>
+    struct has_type_info<NativeInterface, std::void_t<
+        typename NativeInterface::TypeInfo,
+        typename NativeInterface::TypeInfo::baseType,
+        decltype(&NativeInterface::TypeInfo::name),
+        decltype(&NativeInterface::TypeInfo::revision)
+        >> : std::true_type {};
+
+    // We need to wrap the instantiation of has_type_info in a
+    // function friended by TypeInfo, otherwise MSVC will not
+    // let us evaluate TypeInfo in the template arguments.
     template <typename NativeInterface>
-    struct TypeInfo : private NativeInterface
+    bool constexpr hasTypeInfo()
     {
-        static constexpr char const *name() { return NativeInterface::TypeInfo::name; }
-        static constexpr int revision() { return NativeInterface::TypeInfo::revision; }
+        return has_type_info<NativeInterface>::value;
+    }
 
-        template<typename BaseType>
-        static constexpr bool isCompatibleWith =
-            std::is_base_of<typename NativeInterface::TypeInfo::baseType, BaseType>::value;
+    template <typename NativeInterface>
+    struct TypeInfo
+    {
+        // To ensure SFINAE works for hasTypeInfo we can't use it in a constexpr
+        // variable that also includes an expression that relies on the type
+        // info. This helper variable is okey, as it it self contained.
+        static constexpr bool haveTypeInfo = hasTypeInfo<NativeInterface>();
+
+        // We can then use the helper variable in a constexpr condition in a
+        // function, which does not break SFINAE if haveTypeInfo is false.
+        template <typename BaseType>
+        static constexpr bool isCompatibleHelper()
+        {
+            if constexpr (haveTypeInfo)
+                return std::is_base_of<typename NativeInterface::TypeInfo::baseType, BaseType>::value;
+            else
+                return false;
+        }
+
+        // MSVC however doesn't like constexpr functions in enable_if_t conditions,
+        // so we need to wrap it yet again in a constexpr variable. This is fine,
+        // as all the SFINAE magic has been resolved at this point.
+        template <typename BaseType>
+        static constexpr bool isCompatibleWith = isCompatibleHelper<BaseType>();
+
+        // The revision and name accessors are not used in enable_if_t conditions,
+        // so we can leave them as constexpr functions. As this class template is
+        // friended by TypeInfo we can access the protected members of TypeInfo.
+        static constexpr int revision()
+        {
+            if constexpr (haveTypeInfo)
+                return NativeInterface::TypeInfo::revision;
+            else
+                return 0;
+        }
+
+        static constexpr char const *name()
+        {
+            if constexpr (haveTypeInfo)
+                return NativeInterface::TypeInfo::name;
+            else
+                return nullptr;
+        }
     };
 
+    // Wrapper type to make the error message in case
+    // of incompatible interface types read better.
+    template <typename I>
+    struct NativeInterface : TypeInfo<I> {};
+
     template <typename T>
     Q_NATIVE_INTERFACE_IMPORT void *resolveInterface(const T *that, const char *name, int revision);
 
     Q_CORE_EXPORT Q_DECLARE_LOGGING_CATEGORY(lcNativeInterface)
-}
+
+} // QNativeInterface::Private
 
 // Declares an accessor for the native interface
-#define QT_DECLARE_NATIVE_INTERFACE_ACCESSOR \
-    template <typename I> \
-    I *nativeInterface() const \
+#define QT_DECLARE_NATIVE_INTERFACE_ACCESSOR(T) \
+    template <typename NativeInterface, typename TypeInfo = QNativeInterface::Private::NativeInterface<NativeInterface>, \
+    typename BaseType = T, std::enable_if_t<TypeInfo::template isCompatibleWith<T>, bool> = true> \
+    NativeInterface *nativeInterface() const \
     { \
-        using T = std::decay_t<decltype(*this)>; \
-        using NativeInterface = QNativeInterface::Private::TypeInfo<I>; \
-        static_assert(NativeInterface::template isCompatibleWith<T>, \
-            "T::nativeInterface<I>() requires that native interface I is compatible with T"); \
-        \
-        return static_cast<I*>(QNativeInterface::Private::resolveInterface(this, \
-            NativeInterface::name(), NativeInterface::revision())); \
+        return static_cast<NativeInterface*>( \
+            QNativeInterface::Private::resolveInterface(this, \
+                TypeInfo::name(), TypeInfo::revision())); \
     }
 
 // Provides a definition for the interface destructor
@@ -134,6 +206,8 @@ namespace QNativeInterface::Private {
                 revision, TypeInfo<NativeInterface>::revision(), name); \
             return nullptr; \
         } \
+    } else { \
+        qCDebug(lcNativeInterface, "No match for requested interface name %s", name); \
     }
 
 QT_END_NAMESPACE