Report type names in return value policy-related cast exceptions (#1965)

include/pybind11/cast.h

@@ -533,9 +533,17 @@ public:
             case return_value_policy::copy:
                 if (copy_constructor)
                     valueptr = copy_constructor(src);
-                else
-                    throw cast_error("return_value_policy = copy, but the "
-                                     "object is non-copyable!");
+                else {
+#if defined(NDEBUG)
+                    throw cast_error("return_value_policy = copy, but type is "
+                                     "non-copyable! (compile in debug mode for details)");
+#else
+                    std::string type_name(tinfo->cpptype->name());
+                    detail::clean_type_id(type_name);
+                    throw cast_error("return_value_policy = copy, but type " +
+                                     type_name + " is non-copyable!");
+#endif
+                }
                 wrapper->owned = true;
                 break;
 
@@ -544,9 +552,18 @@ public:
                     valueptr = move_constructor(src);
                 else if (copy_constructor)
                     valueptr = copy_constructor(src);
-                else
-                    throw cast_error("return_value_policy = move, but the "
-                                     "object is neither movable nor copyable!");
+                else {
+#if defined(NDEBUG)
+                    throw cast_error("return_value_policy = move, but type is neither "
+                                     "movable nor copyable! "
+                                     "(compile in debug mode for details)");
+#else
+                    std::string type_name(tinfo->cpptype->name());
+                    detail::clean_type_id(type_name);
+                    throw cast_error("return_value_policy = move, but type " +
+                                     type_name + " is neither movable nor copyable!");
+#endif
+                }
                 wrapper->owned = true;
                 break;
 

tests/test_copy_move.py

@@ -5,13 +5,13 @@ from pybind11_tests import copy_move_policies as m
 def test_lacking_copy_ctor():
     with pytest.raises(RuntimeError) as excinfo:
         m.lacking_copy_ctor.get_one()
-    assert "the object is non-copyable!" in str(excinfo.value)
+    assert "is non-copyable!" in str(excinfo.value)
 
 
 def test_lacking_move_ctor():
     with pytest.raises(RuntimeError) as excinfo:
         m.lacking_move_ctor.get_one()
-    assert "the object is neither movable nor copyable!" in str(excinfo.value)
+    assert "is neither movable nor copyable!" in str(excinfo.value)
 
 
 def test_move_and_copy_casts():
@@ -98,7 +98,7 @@ def test_private_op_new():
 
     with pytest.raises(RuntimeError) as excinfo:
         m.private_op_new_value()
-    assert "the object is neither movable nor copyable" in str(excinfo.value)
+    assert "is neither movable nor copyable" in str(excinfo.value)
 
     assert m.private_op_new_reference().value == 1