QTypeInfo: Handle T::value_type being equal to T

Fix operator checks for containers whose value_type equals themselves.
It does  not make sense to recurse on value_type in that case. Thanks to
std::disjunction having short-circuiting semantics, we can avoid that
issue by checking first whether T is T::value_type.

As a drive-by, check for value_type typedef before checking for
begin/end in is_container. This works around an issue in gcc <= 8.1,
which fails to correctly SFINAE the case where begin and end are private
methods.

Pick-to: 6.0 6.1
Fixes: QTBUG-89456
Change-Id: I27305a7cfe050f13a279c07f00bc229c01daa25b
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>

src/corelib/global/qtypeinfo.h

@@ -228,8 +228,8 @@ template <typename, typename = void>
 struct is_container : std::false_type {};
 template <typename T>
 struct is_container<T, std::void_t<
-        std::is_convertible<decltype(std::declval<T>().begin() != std::declval<T>().end()), bool>,
-        typename T::value_type
+        typename T::value_type,
+        std::is_convertible<decltype(std::declval<T>().begin() != std::declval<T>().end()), bool>
 >> : std::true_type {};
 
 
@@ -259,7 +259,11 @@ struct expand_operator_equal_container : expand_operator_equal_tuple<T> {};
 // if T::value_type exists, check first T::value_type, then T itself
 template<typename T>
 struct expand_operator_equal_container<T, true> :
-        std::conjunction<expand_operator_equal<typename T::value_type>, expand_operator_equal_tuple<T>> {};
+        std::conjunction<
+        std::disjunction<
+            std::is_same<T, typename T::value_type>, // avoid endless recursion
+            expand_operator_equal<typename T::value_type>
+        >, expand_operator_equal_tuple<T>> {};
 
 // recursively check the template arguments of a tuple like object
 template<typename ...T>
@@ -295,7 +299,12 @@ template<typename T, bool>
 struct expand_operator_less_than_container : expand_operator_less_than_tuple<T> {};
 template<typename T>
 struct expand_operator_less_than_container<T, true> :
-        std::conjunction<expand_operator_less_than<typename T::value_type>, expand_operator_less_than_tuple<T>> {};
+        std::conjunction<
+            std::disjunction<
+                std::is_same<T, typename T::value_type>,
+                expand_operator_less_than<typename T::value_type>
+            >, expand_operator_less_than_tuple<T>
+        > {};
 
 template<typename ...T>
 using expand_operator_less_than_recursive = std::conjunction<expand_operator_less_than<T>...>;

tests/auto/corelib/kernel/qmetatype/tst_qmetatype.cpp

@@ -2128,5 +2128,15 @@ void tst_QMetaType::constexprMetaTypeIds()
     Q_UNUSED(metaType);
 }
 
+struct S {
+  using value_type = S; // used to cause compilation error with Qt6
+  int begin();
+  int end();
+};
+
+// should not cause a compilation failure
+// used to cause issues due to S being equal to S::value_type
+Q_DECLARE_METATYPE(S)
+
 QTEST_MAIN(tst_QMetaType)
 #include "tst_qmetatype.moc"