Short live q20::iter_reference_t

Another building block for QSpan.

Task-number: QTBUG-108124
Change-Id: Ic7205ec693d953f6b054282380e87e79dead8816
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/global/q20iterator.h

@@ -39,6 +39,16 @@ namespace q20 {
 #endif
 } // namespace q20
 
+// like q20::iter_reference_t
+namespace q20 {
+#ifdef __cpp_lib_ranges
+    using std::iter_reference_t;
+#else
+    template <typename Dereferencable> // unconstrained (constraint requires concepts)
+    using iter_reference_t = decltype(*std::declval<Dereferencable&>());
+#endif // __cpp_lib_ranges
+} // namespace q20
+
 QT_END_NAMESPACE
 
 #endif /* Q20ITERATOR_H */