Constrain the debug stream operators for containers

Check that we can successfully instantiate the debug
stream operator for a container before we actually try.

This is required so we can automate registration of debug
stream operators with QMetaType.

Change-Id: I3943e7a443751d250c33b2ca1b9cf29207cfe6c4
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

1 files changed, 19 insertions, 0 deletions

diff --git a/src/corelib/global/qtypeinfo.h b/src/corelib/global/qtypeinfo.h
index be21526339..2f7bb414b5 100644
--- a/src/corelib/global/qtypeinfo.h
+++ b/src/corelib/global/qtypeinfo.h
@@ -48,6 +48,8 @@
 
 QT_BEGIN_NAMESPACE
 
+class QDebug;
+
 /*
    QTypeInfo     - type trait functionality
 */
@@ -445,6 +447,23 @@ using compare_eq_result = std::enable_if_t<std::conjunction_v<QTypeTraits::has_o
 template <typename ...T>
 using compare_lt_result = std::enable_if_t<std::conjunction_v<QTypeTraits::has_operator_less_than<T>...>, bool>;
 
+namespace detail {
+
+template<typename T>
+const T const_value();
+template<typename T>
+T &reference();
+
+}
+
+template <typename Stream, typename, typename = void>
+struct has_ostream_operator : std::false_type {};
+template <typename Stream, typename T>
+struct has_ostream_operator<Stream, T, std::void_t<decltype(detail::reference<Stream>() << detail::const_value<T>())>>
+        : std::true_type {};
+template <typename Stream, typename T>
+constexpr bool has_ostream_operator_v = has_ostream_operator<Stream, T>::value;
+
 }