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More specific inline dict type inference can fail on 1.17.0 #19474

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More specific inline dict type inference can fail on 1.17.0#19474
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bugmypy got something wrongtopic-inferenceWhen to infer types or require explicit annotations

Description

@markhobson
markhobson
opened on Jul 17, 2025

Bug Report

More specific inline dict type inference can fail on 1.17.0.

To Reproduce

The following fails on 1.17.0 but is fine on 1.16.1 (the playground link uses master as 1.17.0 isn't available there):
https://mypy-play.net/?mypy=master&python=3.13&gist=7e77b89fe5f9cb7e548c4ef83b3783c1

Revealing the type for the inline dict gives:

  • 1.16.1: builtins.dict[builtins.str, Union[typing.Collection[builtins.str], None]]
  • 1.17.0: builtins.dict[builtins.str, builtins.dict[builtins.str, builtins.list[Any] | None] | builtins.dict[builtins.str, builtins.str] | builtins.str | None]

Expected Behavior

No mypy errors.

Actual Behavior

main.py:11: error: Dict entry 0 has incompatible type "str": "None"; expected "str": "str"  [dict-item]
main.py:12: error: Dict entry 1 has incompatible type "str": "list[Never]"; expected "str": "str"  [dict-item]
Found 2 errors in 1 file (checked 1 source file)

Your Environment

  • Mypy version used: 1.17.0
  • Mypy command-line flags: N/A
  • Mypy configuration options from mypy.ini (and other config files): N/A
  • Python version used: 3.13.5

Activity

  1. markhobson
    added
    bugmypy got something wrong
    on Jul 17, 2025
  2. markhobson
    added a commit that references this issue on Jul 17, 2025

    Upgrade Mypy to 1.17.x

    ea0aa24

  3. sterliakov commented on Jul 17, 2025

    @sterliakov
    sterliakov
    on Jul 17, 2025
    Collaborator

    Bisects to #19158; it might have only uncovered an existing issue and not be the direct cause.

  4. sterliakov
    added
    topic-inferenceWhen to infer types or require explicit annotations
    on Jul 17, 2025

  5. hauntsaninja commented on Jul 17, 2025

    @hauntsaninja
    hauntsaninja
    on Jul 17, 2025
    Collaborator

    Yeah, it just exposed existing issue. The following will repro on 1.16 as well:

    from typing import Any


json: Any
if json == {
    "x": None,
    "x": "y",
    "x": {
        "x": "y",
    },
    "x": {
        "x": None,
        "x": [],
    },
}:
    pass
  6. markhobson
    added a commit that references this issue on Aug 1, 2025

    Upgrade Mypy to 1.17.x

    942d449
  7. markhobson
    added a commit that references this issue on Sep 19, 2025

    Upgrade Mypy to 1.18.x

    9644cdd
  8. markhobson
    added a commit that references this issue on Oct 9, 2025

    Upgrade Mypy to 1.18.x

    8c7e2da
  9. markhobson
    added 2 commits that reference this issue on Dec 1, 2025

    Upgrade Mypy to 1.18.x

    41ffdd3

    Upgrade Mypy to 1.19.x

    7be5187
  10. markhobson
    added 2 commits that reference this issue on Feb 17, 2026

    GH-67: Upgrade Mypy to 1.19.x

    fd313c6

    GH-67: Upgrade Mypy to 1.19.x

    3a91ea3
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