python / python/mypy

overload with wrong return type is ignored

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#17,857 4 comments 1 reaction 0 assignees View on GitHub

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feature topic-overloads
Dominant language
Python
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Description

Bug Report

When one of the alternatives of an overload has a return type T which is a subtype of the return type of the annotation U but that the body of the function returns a type V which is a subtype of U but not T, no error is reported

To Reproduce

from typing import overload

@overload
def f(a: int) -> str:
    ...

# this is completely wrong
@overload
def f(a: str) -> bool:
    ...


def f(a: str | int) -> str | bool:
    return "Hello"

reveal_type(f("some string"))

https://mypy-play.net/?mypy=latest&python=3.12&flags=strict&gist=83c5a353e087d8bbe03d542e7340b4b5

Expected Behavior

mypy should complain that the body of the function returns an str which is incompatible with type bool (overload 2)

More generally, for each overload signature, mypy should check that the body of the function typechecks with the overload signature.

Actual Behavior

mypy reports no error

main.py:16: note: Revealed type is "builtins.bool"
Success: no issues found in 1 source file

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start with the Python reproducer shown in main.py and run it with the linked mypy-play strict configuration. Trace how overload signatures are checked against the implementation return type. Done means mypy reports the incompatible str return for overload 2 and the regression is covered by a test.

Written by the indexing model from the issue text.

Assessment

Tech stack
python
Domain
devtools
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
38/100

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