Union of dicts with heterogeneous value types

[harahu]

I'm running into a type checking error where I'm unsure whether what I'm looking at is an issue in mypy or an issue in my understanding of python type annotations.

See: https://mypy-play.net/?mypy=master&python=3.13&flags=show-error-codes&gist=2643cb3ff1ea956a1c4ed0869140d46d

d1: dict[str, str] = {}
d2: dict[str, str | None] = {}

d3 = d1 | d2
reveal_type(d3)  # builtins.dict[builtins.str, Union[builtins.str, None]], aka dict[str, str | None]

# This is fine
d4: dict[str, str | None] = d3  # aka d1 | d2

# This isn't?
d5: dict[str, str | None] = d1 | d2

I cannot figure out why I'm not allowed to do d5: dict[str, str | None] = d1 | d2 in this case.

[erictraut]

I think this is an issue with mypy. The code looks fine to me, and pyright accepts it without any errors.