Incorrect "redundant cast" between total and partial dicts

[alicederyn]

Bug Report

Casting a total dict to an equivalent non-total dict incorrectly warns that the cast is redundant.

To Reproduce

class Permissive(TypedDict, total=False):
  id: str

class Restrictive(TypedDict):
  id: str

def convert(d: Restrictive) -> Permissive:
  return cast(Permissive, d)

Expected Behavior

No mypy errors/warnings

Actual Behavior

error: Redundant cast to "Permissive"

The mypy docs state "a total TypedDict is not valid when a partial one is expected", so this seems incorrect, and indeed if I remove the cast, I get:

Incompatible return value type (got "Restrictive", expected "Permissive")

Your Environment

• Mypy version used: Checked with all versions between 0.812 and 0.931 inclusive
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files):

python_version = "3.9"
namespace_packages = true
explicit_package_bases = true
strict = true

• Python version used: 3.9
• Operating system and version: docker, looks like RHEL 7.9, running within Ubuntu 10 on WSL 2

[alicederyn]

Apologies if this is a duplicate! I searched but the existing issues I found seemed like they were about other redundant cast bugs.

[A5rocks]

Note that it should be a redundant cast under #12142... I'm not sure if it will be accepted though.

[sodul]

We recently ran in a similar issue with boto stubs where strict output data from boto calls could not be re-used as input data for other boto calls where missing keys are allowed.

youtype/mypy_boto3_builder#208

The fix was apparently to remove the Output types and only use the types with Total=False, but this is a bit of a lie since the output types are supposed to contain all the keys.

Is there are better alternative than using cast() or always use total=False even if untrue?

[erictraut]

It appears this was fixed in mypy 0.981.

[hauntsaninja]

Fixed in #13311