[cppyy] Handle polymorphic types when converting from class fields

fixes #16062

Author: Vipul-Cariappa

bindings/pyroot/cppyy/CPyCppyy/src/Converters.cxx

@@ -2183,9 +2183,10 @@ template <bool ISCONST>
 PyObject* CPyCppyy::InstancePtrConverter<ISCONST>::FromMemory(void* address)
 {
 // construct python object from C++ instance read at <address>
+    Cppyy::TCppScope_t actual_class = Cppyy::GetActualClass(fClass, *(void**)address);
     if (ISCONST)
-        return BindCppObject(*(void**)address, fClass);                   // by pointer value
-    return BindCppObject(address, fClass, CPPInstance::kIsReference);     // modifiable
+        return BindCppObject(*(void**)address, actual_class);                   // by pointer value
+    return BindCppObject(address, actual_class, CPPInstance::kIsReference);     // modifiable
 }
 
 //----------------------------------------------------------------------------

bindings/pyroot/cppyy/cppyy/test/test_datatypes.py

@@ -2370,3 +2370,38 @@ def test50_int8_uint8_global_arrays(self):
 
         assert [ns.test[i]  for i in range(6)] == [-0x12, -0x34, -0x56, -0x78, 0x0, 0x0]
         assert [ns.utest[i] for i in range(6)] == [ 0x12,  0x34,  0x56,  0x78, 0x0, 0x0]
+
+    def test51_polymorphic_types_in_maps(self):
+        """Auto down-cast polymorphic types in maps"""
+        import cppyy
+        from cppyy import gbl
+
+        cppyy.cppdef(
+            """
+        namespace PolymorphicMaps {
+        struct Base {
+            int x;
+            Base(int x) : x(x) {}
+            virtual ~Base() {}
+        } b(1);
+
+        struct Derived : public Base {
+            int y;
+            Derived(int i) : Base(i), y(i) {}
+        } d(1);
+
+        std::map<int, Base*> getBaseMap() {
+            std::map<int, Base*> m;
+            m[1] = &b;
+            m[2] = &d;
+            return m;
+        }
+        }  // namespace PolymorphicMaps
+        """
+        )
+
+        for k, v in gbl.PolymorphicMaps.getBaseMap():
+            if k == 1:
+                assert type(v) == gbl.PolymorphicMaps.Base
+            else:
+                assert type(v) == gbl.PolymorphicMaps.Derived