Revert "Use a dictionary function map"

This reverts commit a38544b.

Author: tomschr

src/docbuild/config/merge.py

@@ -28,16 +28,8 @@ def deep_merge(*dcts: Mapping[str, Any]) -> dict[str, Any]:
     if not dcts:
         return {}
 
-    # Initial deep copy of the base dictionary
     result = deepcopy(dict(dcts[0]))
 
-    # Handlers for specific types (excluding Mapping which needs special handling)
-    funcmap = {
-        (list, list): lambda e, v: e + deepcopy(v),
-        (tuple, tuple): lambda e, v: e + deepcopy(v),
-        (set, set): lambda e, v: e | deepcopy(v),
-    }
-
     for src in dcts[1:]:
         stack: list[tuple[dict[str, Any], Mapping[str, Any]]] = [(result, src)]
 
@@ -51,19 +43,26 @@ def deep_merge(*dcts: Mapping[str, Any]) -> dict[str, Any]:
 
                 existing = dest[key]
 
-                # Special handling for Mappings (recursive merge)
+                # Mapping → merge deeper
                 if isinstance(existing, Mapping) and isinstance(value, Mapping):
+                    # Ensure we are working with a dict for the merge target
+                    # This avoids nested specialized types if they support item assignment
                     if type(existing) is not dict:
                         existing = dict(existing)
                         dest[key] = existing
                     stack.append((existing, value))
-                    continue
 
-                # Use funcmap for other types
-                # Using get() on dict with tuple key avoids the loop
-                handler = funcmap.get((type(existing), type(value)))
-                if handler:
-                    dest[key] = handler(existing, value)
+                # Lists / tuples → concatenate
+                elif (isinstance(existing, list) and isinstance(value, list)) or (
+                    isinstance(existing, tuple) and isinstance(value, tuple)
+                ):
+                    dest[key] = existing + deepcopy(value)  # type: ignore[operator]
+
+                # Sets → union
+                elif isinstance(existing, set) and isinstance(value, set):
+                    dest[key] = existing | deepcopy(value)
+
+                # Fallback → overwrite
                 else:
                     dest[key] = deepcopy(value)