Wacky `dict` `reveal_type` moment

```py
a = reveal_type({ # Revealed type is "dict['C' | 'D', None]"
"A": None,
"B": None,
"C": None,
"D": None,
})
reveal_type(a) # Revealed type is "dict[str, None]"
```

Start by running the provided snippet through basedmypy and compare the inline and subsequent reveal_type results. Trace the dictionary-literal type inference to determine why the two outputs differ; done means the behavior is explained or corrected and covered by a regression check.

Written by the indexing model from the issue text.