merge_config loses nested settings

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Final diff

diff --git a/settings.py b/settings.py
index 8b119fb..3d3580d 100644
--- a/settings.py
+++ b/settings.py
@@ -1,2 +1,17 @@
+import copy
+
+
 def merge_config(base: dict, override: dict) -> dict:
-    return {**base, **override}
+    """Return a new dict with override merged into base.
+
+    Nested dicts are merged recursively. Any other value in override replaces
+    the value in base. Neither input is modified, and the result does not share
+    mutable state with either input.
+    """
+    result = copy.deepcopy(base)
+    for key, value in override.items():
+        if isinstance(result.get(key), dict) and isinstance(value, dict):
+            result[key] = merge_config(result[key], value)
+        else:
+            result[key] = copy.deepcopy(value)
+    return result
diff --git a/test_settings.py b/test_settings.py
index 52bc84b..4393b20 100644
--- a/test_settings.py
+++ b/test_settings.py
@@ -1,3 +1,5 @@
+import copy
+
 from settings import merge_config
 
 
@@ -7,3 +9,39 @@ def test_flat_merge():
 
 def test_override_wins():
     assert merge_config({"a": 1}, {"a": 2}) == {"a": 2}
+
+
+def test_nested_merge_keeps_base_keys():
+    base = {"db": {"host": "a", "port": 1}}
+    override = {"db": {"port": 2}}
+    assert merge_config(base, override) == {"db": {"host": "a", "port": 2}}
+
+
+def test_non_dict_override_replaces_dict_base():
+    assert merge_config({"db": {"host": "a"}}, {"db": None}) == {"db": None}
+    assert merge_config({"db": {"host": "a"}}, {"db": [1, 2]}) == {"db": [1, 2]}
+
+
+def test_dict_override_replaces_non_dict_base():
+    assert merge_config({"db": 5}, {"db": {"port": 2}}) == {"db": {"port": 2}}
+
+
+def test_keys_only_in_one_side_are_kept():
+    base = {"a": 1, "nested": {"x": 1}}
+    override = {"b": 2, "nested": {"y": 2}}
+    assert merge_config(base, override) == {"a": 1, "b": 2, "nested": {"x": 1, "y": 2}}
+
+
+def test_inputs_are_not_modified():
+    base = {"db": {"host": "a", "port": 1}, "list": [1]}
+    override = {"db": {"port": 2, "opts": {"ssl": True}}}
+    base_copy = copy.deepcopy(base)
+    override_copy = copy.deepcopy(override)
+
+    result = merge_config(base, override)
+
+    assert base == base_copy
+    assert override == override_copy
+    assert result is not base and result is not override
+    result["db"]["opts"]["ssl"] = False
+    assert override == override_copy