Fixed Merging into a MISSING Structured config

news/269.bugfix

@@ -0,0 +1 @@
+Merging into a MISSING Structured config node expands the node first to ensure the result is legal

news/271.bugfix

@@ -0,0 +1 @@
+Fix merging into a config with a read only node if merge is not mutating that node

omegaconf/basecontainer.py

@@ -12,14 +12,14 @@
     _convert_to_omegaconf_container,
     _get_value,
     _is_interpolation,
+    _resolve_optional,
+    get_ref_type,
     get_value_kind,
     get_yaml_loader,
+    is_dict_annotation,
     is_primitive_container,
     is_primitive_dict,
     is_structured_config,
-    get_ref_type,
-    is_dict_annotation,
-    _resolve_optional,
 )
 from .base import Container, ContainerMetadata, Node
 from .errors import MissingMandatoryValue, ReadonlyConfigError, ValidationError
@@ -232,8 +232,6 @@ def pretty(self, resolve: bool = False, sort_keys: bool = False) -> str:
     @staticmethod
     def _map_merge(dest: "BaseContainer", src: "BaseContainer") -> None:
         """merge src into dest and return a new copy, does not modified input"""
-        from omegaconf import OmegaConf
-
         from .dictconfig import DictConfig
         from .listconfig import ListConfig
         from .nodes import ValueNode

tests/test_merge.py

@@ -1,5 +1,6 @@
-from typing import Any, Tuple
 from dataclasses import dataclass
+from typing import Any, Tuple
+
 import pytest
 
 from omegaconf import (
@@ -217,22 +218,6 @@ def test_merge(inputs: Any, expected: Any) -> None:
             OmegaConf.merge(*configs)
 
 
-"""
-from omegaconf import OmegaConf, MISSING
-from dataclasses import dataclass
-
-@dataclass
-class A:
-    a : int = 10
-
-@dataclass
-class B:
-    x : A = MISSING
-
-OmegaConf.merge(B, {"x": {}})
-"""
-
-
 def test_merge_error_retains_type() -> None:
     cfg = OmegaConf.structured(ConcretePlugin)
     with pytest.raises(ValidationError):