[python] Add regression test for #11854

Author: vepadulano

bindings/pyroot/pythonizations/test/CMakeLists.txt

@@ -191,3 +191,5 @@ if(NOT MSVC OR CMAKE_SIZEOF_VOID_P EQUAL 8 OR win_broken_tests)
     # https://github.com/root-project/root/issues/9809
     ROOT_ADD_PYUNITTEST(pyroot_array_numpy_views array_conversions.py PYTHON_DEPS numpy)
 endif()
+
+ROOT_ADD_PYUNITTEST(gh_11854 gh_11854.py)

bindings/pyroot/pythonizations/test/gh_11854.py

@@ -0,0 +1,40 @@
+import unittest
+
+import ROOT
+
+
+class GH11854(unittest.TestCase):
+    """Regression test for https://github.com/root-project/root/issues/11854"""
+
+    def test_gh_11854(self):
+
+        ROOT.gInterpreter.Declare(r"""
+        #ifndef GH11854_HELPER
+        #define GH11854_HELPER
+
+        template <typename T>
+        struct GH11854Helper {
+
+            std::size_t operator() () const {
+                const std::size_t res = 0;
+                res = T{0, 0}.size();
+                return res;
+            }
+
+        };
+
+        template <typename H>
+        std::size_t call_gh11854_helper(const H &helper) {
+            return helper();
+        }
+
+        #endif // GH11854_HELPER
+        """)
+
+        helper = ROOT.GH11854Helper[ROOT.std.vector["double"]]()
+        with self.assertRaisesRegex(TypeError, "cannot assign to variable 'res' with const-qualified type 'const std::size_t'"):
+            ROOT.call_gh11854_helper(helper)
+
+
+if __name__ == '__main__':
+    unittest.main()