QMinimalFlatSet: fix UB (using op< on pointers) when is_pointer<value_type>

Using operator< on pointers that are not part of the same array is
UB. We need to use std::less to get a total order, so do that.

The QMinimalFlatSet copy in QtDeclarative is not affected, because
it's only ever instantiated with value_type int.

Change-Id: Ic8cd4852505f3d3ab57039ce26064ed47cac0deb
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

src/corelib/tools/qminimalflatset_p.h

@@ -29,6 +29,7 @@
 #endif
 
 #include <algorithm> // for std::lower_bound
+#include <functional> // for std::less
 
 QT_BEGIN_NAMESPACE
 
@@ -120,8 +121,10 @@ private:
             bool exists;
         };
 
-        const auto it = std::lower_bound(c.cbegin(), c.cend(), v);
-        return R{it, it != c.cend() && !(v < *it)};
+        auto cmp = std::less<value_type>{};
+
+        const auto it = std::lower_bound(c.cbegin(), c.cend(), v, cmp);
+        return R{it, it != c.cend() && !cmp(v, *it)};
     }
 
 #ifdef QMINIMAL_FLAT_SET_DEBUG