QPermission: don't hide SFINAE constraints

Like done elsewhere (e.g. in QStringView), formulate the constaint in
if_x<T> = true from, and let qdoc see it. Then document the constraint
in prose, too.

As a drive-by, use C++17 variable templates instead of
std::bool_value, and document that data() returns a
default-constructed T on failure (which is indistinguishable from a
successful call that happens to return a default-constructed T, so we
should probably return an optional<T> here, but that's orthogonal to
the change at hand).

Change-Id: I0584ce3f4febd619d3966afeb6244b11b167cd42
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
(cherry picked from commit 6ac9d46a36f1b6b921efdcb47d7b7241d2396897)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/kernel/qpermissions.cpp

@@ -229,22 +229,32 @@ Q_LOGGING_CATEGORY(lcPermissions, "qt.permissions", QtWarningMsg);
 */
 
 /*!
-    \fn template <typename Type> QPermission::QPermission(const Type &type)
+    \fn template <typename T, if_permission<T>> QPermission::QPermission(const T &type)
 
     Constructs a permission from the given \l{typed permission} \a type.
 
     You do not need to construct this type explicitly, as the type is automatically
     used when checking or requesting permissions.
+
+    This constructor participates in overload resolution only if \c T is one of
+    the \l{typed permission} classes:
+
+    \annotatedlist permissions
 */
 
 /*!
-    \fn template <typename Type> Type QPermission::data() const
+    \fn template <typename T, if_permission<T>> T QPermission::data() const
 
-    Returns the \l{typed permission} of type \c Type.
+    Returns the \l{typed permission} of type \c T.
 
-    The type must match the type that was originally used to request
-    the permission. Use type() for dynamically choosing which typed
-    permission to request.
+    If the type doesn't match the type that was originally used to request the
+    permission, returns a default-constructed \c T. Use type() for dynamically
+    choosing which typed permission to request.
+
+    This function participates in overload resolution only if \c T is one of
+    the \l{typed permission} classes:
+
+    \annotatedlist permissions
 */
 
 /*!

src/corelib/kernel/qpermissions.h

@@ -34,31 +34,24 @@ class QPermission
     Q_GADGET_EXPORT(Q_CORE_EXPORT)
 
     template <typename T, typename Enable = void>
-    struct is_permission : public std::false_type {};
+    static constexpr inline bool is_permission_v = false;
 
     template <typename T>
-    struct is_permission<T, typename T::QtPermissionHelper> : public std::true_type {};
+    static constexpr inline bool is_permission_v<T, typename T::QtPermissionHelper> = true;
 
+    template <typename T>
+    using if_permission = std::enable_if_t<is_permission_v<T>, bool>;
 public:
     explicit QPermission() = default;
 
-#ifdef Q_QDOC
-    template <typename Type>
-    QPermission(const Type &type);
-#else
-    template <typename T, std::enable_if_t<is_permission<T>::value, bool> = true>
+    template <typename T, if_permission<T> = true>
     QPermission(const T &t) : m_data(QVariant::fromValue(t)) {}
-#endif
 
     Qt::PermissionStatus status() const { return m_status; }
 
     QMetaType type() const { return m_data.metaType(); }
 
-#ifdef Q_QDOC
-    template <typename Type>
-    Type data() const;
-#else
-    template <typename T, std::enable_if_t<is_permission<T>::value, bool> = true>
+    template <typename T, if_permission<T> = true>
     T data() const
     {
         if (auto p = data(QMetaType::fromType<T>()))
@@ -66,7 +59,6 @@ public:
         else
             return T{};
     }
-#endif
 
 #ifndef QT_NO_DEBUG_STREAM
     friend Q_CORE_EXPORT QDebug operator<<(QDebug debug, const QPermission &);