# Bug Review: merge_user_prefs Function

## Critical Issues

### 1. **Mutation of Input Data (Docstring Violation)**
The function claims "Does not mutate inputs" but it directly mutates `default_prefs`:
```python
result = default_prefs  # This is a reference, not a copy
result[key] = value    # Mutates the original dict
```
This violates the documented contract and is a serious bug.

**Fix:** Create a copy first:
```python
result = default_prefs.copy()
```

### 2. **Shallow Copy Only**
If `default_prefs` contains nested mutable objects (dicts, lists), `.copy()` only creates a shallow copy. Mutations to nested structures would still affect the original. For nested safety, use:
```python
result = copy.deepcopy(default_prefs)
```

## Additional Issues

### 3. **Silent Filtering of None Values**
The condition `if value is not None:` means None values in `user_overrides` are silently ignored rather than explicitly setting keys to None. This may be intentional, but it's worth documenting—the docstring should clarify this behavior.

### 4. **No Type Validation**
The function assumes both arguments are dicts but doesn't validate this. If passed non-dict types, it will fail with unclear error messages.

## Summary
The primary bug is mutating `default_prefs` despite the docstring promise. Always create a copy before modification. Secondary concern: shallow vs. deep copy depending on data structure complexity.