Merging a missing list onto an existing one makes the target missing

news/306.bugfix

@@ -0,0 +1 @@
+Merging a missing list onto an existing one makes the target missing

omegaconf/basecontainer.py

@@ -336,8 +336,11 @@ def _merge_with(
                 else:
                     self.__dict__["_content"].clear()
 
-                for item in other:
-                    self.append(item)
+                if other._is_missing():
+                    self._set_value("???")
+                else:
+                    for item in other:
+                        self.append(item)
 
                 # explicit flags on the source config are replacing the flag values in the destination
                 for flag, value in other._metadata.flags.items():

tests/test_merge.py

@@ -1,4 +1,4 @@
-from typing import Any, Tuple
+from typing import Any, MutableMapping, MutableSequence, Tuple
 
 import pytest
 
@@ -50,6 +50,11 @@
             {"a": {"b": 10}},
             id="dict_merge_into_missing",
         ),
+        pytest.param(
+            ({"a": {"b": 10}}, {"a": "???"}),
+            {"a": "???"},
+            id="dict_merge_missing_onto",
+        ),
         # lists
         (([1, 2, 3], [4, 5, 6]), [4, 5, 6]),
         (([[1, 2, 3]], [[4, 5, 6]]), [[4, 5, 6]]),
@@ -62,6 +67,16 @@
             {"a": [1, 2, 3]},
             id="list_merge_into_missing",
         ),
+        pytest.param(
+            ({"a": [1, 2, 3]}, {"a": "???"}),
+            {"a": "???"},
+            id="list_merge_missing_onto",
+        ),
+        pytest.param(
+            ([1, 2, 3], ListConfig(content=MISSING)),
+            ListConfig(content=MISSING),
+            id="list_merge_missing_onto2",
+        ),
         # Interpolations
         # value interpolation
         pytest.param(
@@ -205,7 +220,9 @@
 def test_merge(inputs: Any, expected: Any) -> None:
     configs = [OmegaConf.create(c) for c in inputs]
 
-    if isinstance(expected, (dict, list)) or is_structured_config(expected):
+    if isinstance(expected, (MutableMapping, MutableSequence)) or is_structured_config(
+        expected
+    ):
         merged = OmegaConf.merge(*configs)
         assert merged == expected
         # test input configs are not changed.