Fail composition when a dangling override is rescued by an append

A Defaults List override with no earlier Group Default to override correctly
fails on its own, but a matching command line append made the composition
succeed and silently replaced the appended value with the dangling override's.

Stop registering a defaults list's own override when an externally appended
group default of the same key is resolved. The appended entry now composes
without consulting the dangling override, which is then reported unused, so
the composition fails exactly as it does without the append. Command line
overrides of appended groups are registered before traversal and still apply.

Add regression coverage for the dangling override with and without the append.

Fixes #3318

Author: TheSaiEaranti

hydra/_internal/defaults_list.py

@@ -574,16 +574,6 @@ def _create_defaults_tree_impl(
 
     _update_overrides(defaults_list, overrides, parent, interpolated_subtree)
 
-    # An externally appended group default is the one case where a group default
-    # may legally follow an override of the same group in a single defaults list.
-    # The reverse traversal below visits the appended entry before that override,
-    # so map each key to the list's last override for it, allowing the appended
-    # entry to register its override before resolving.
-    list_overrides: Dict[str, GroupDefault] = {}
-    for d in defaults_list:
-        if isinstance(d, GroupDefault) and d.is_override():
-            list_overrides[d.get_override_key()] = d
-
     def add_child(
         child_list: List[Union[InputDefault, DefaultsTreeNode]],
         new_root_: DefaultsTreeNode,
@@ -613,11 +603,6 @@ def add_child(
 
             d.update_parent(parent.get_group_path(), parent.get_final_package())
 
-            if isinstance(d, GroupDefault) and d.is_external_append():
-                pending = list_overrides.get(d.get_override_key())
-                if pending is not None:
-                    overrides.add_override(parent.get_config_path(), pending)
-
             if overrides.is_overridden(d):
                 assert isinstance(d, GroupDefault)
                 overrides.override_default_option(d)

news/3318.bugfix

@@ -0,0 +1 @@
+A Defaults List override that does not target an earlier Group Default now fails to compose even when a command line append introduces a matching group, instead of silently discarding the appended value.

tests/defaults_list/test_defaults_tree.py

@@ -215,11 +215,15 @@ def test_simple_defaults_tree_cases(
         param(
             "group_override_only",
             ["+group1=file1"],
-            DefaultsTreeNode(
-                node=ConfigDefault(path="group_override_only"),
-                children=[GroupDefault(group="group1", value="file2")],
+            raises(
+                ConfigCompositionException,
+                match=re.escape(
+                    dedent("""\
+                        In 'group_override_only': Could not override 'group1'.
+                        Did you mean to override group1?""")
+                ),
             ),
-            id="group_override_only:append_overridden_group",
+            id="group_override_only:append_does_not_rescue_dangling_override",
         ),
     ],
 )
@@ -2625,6 +2629,15 @@ def test_missing_config_errors(
             ),
             id="error_invalid_override",
         ),
+        param(
+            "group_override_only",
+            [],
+            raises(
+                ConfigCompositionException,
+                match="Could not override 'group1'. No match in the defaults list.",
+            ),
+            id="dangling_override_without_append",
+        ),
         param(
             "group_default",
             ["group1@foo=file1"],