False positive [operator] on union of dicts with heterogeneous value types

[harahu]

Bug Report

If I take the union of two dicts with different value types, I cannot assign the result to a variable that is annotated with the resulting type. Doing so causes an error. Assigning to an non-annotated variable does not cause an error, which seems like the correct behavior.

Do I really need the annotation on d5 below? No. But I have a non-toy example of this in a code base where d5 is a type annotated attribute on an object, which results in the same error.

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

To Reproduce

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

Expected Behavior

I expect d5: dict[str, str | None] = d1 | d2 to be legal.

Actual Behavior

error: Unsupported operand types for | ("dict[str, str]" and "dict[str, str | None]") [operator].

Your Environment

• Mypy version used: 1.13.0
• Mypy command-line flags:
• Mypy configuration options from mypy.ini (and other config files):
• Python version used: 3.12

[Aarya2004]

Hey @harahu, I'd love to work on this! Thanks for including the code snippet, I'm looking through the code for ways to make a fix. Let me know if you have any possible solutions, I'll let you know if I have any updates!

[brianschubert]

@Aarya2004 Thanks for your interest in contributing to mypy! Unfortunately, this probably isn't a good issue for a first contribution. This issue has to do with they way mypy applies bidirectional inference, which a) is a very complex part of the code, and b) involves making tradeoffs that are tricky to get right and have a high risk of breaking behavior that mypy users rely on.

I'd recommend taking a look at issue tagged good-first-issue or good-second-issue , which should be a bit more approachable and are a good way to build up to larger issues. You can also check out CONTRIBUTING.md and the Developer Guides for contributing guidelines and tips on where to start.

[Aarya2004]

Thanks for the guidance @brianschubert. I'll take a look at that then!