Warn Return Any now applies to small lambdas

[alanhdu]

Bug Report

Suppose we have the follow code:

from typing import Any

x: Any = [1, 2, 3]
sorted([0, 2, 1], key=lambda i: x[i])

With mypy 0.790, I get an error:

$ mypy --warn-return-any scratch.py
scratch.py:4: error: Returning Any from function declared to return "_SupportsLessThan"
Found 1 error in 1 file (checked 1 source file)

With mypy 0.782, this passes:

$ mypy --warn-return-any scratch.py
Success: no issues found in 1 source file

I'm not actually sure that this is a bug or whether this is an intended feature -- I could see it either way. On one hand, this is technically a case where we are returning any, so warning on this follows the flag name. OTOH though, lambdas are often used for small things where it'd be syntactically cumbersome to add casts.

Your Environment

• Mypy version used: 0.790
• Mypy command-line flags: --warn-return-any
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: Python 3.6
• Operating system and version: Linux

[hauntsaninja]

Thanks for posting your repro! The thing that's changed between 0.782 and 0.790 is the definition of sorted. key used to be a function that returned Any, but is now marked as a function that returns a SupportsLessThan.
This makes it similar to #9590 where itertools.groupby takes a function that returns a typevar (inferred as object).

I agree that I'm not sure to what extent this is intentional, but a) I agree it's kind of annoying, b) as #9590 points out, we don't get the error when aliasing the lambda.