Unsupported converter, only named functions, types and lambdas are currently supported with higher-order function

Bug Report

Mypy succeeds evaluating an attr converter that is an (Any) -> Any function, but fails to evaluate a converter that is returned by a higher-order function that returns an (Any) -> Any function.

To Reproduce

This works as expected:

from attr import define, field


def truncate(val):
    return val[:40]


@define
class Person:
    first_name: str = field(converter=truncate)
    last_name: str = field(converter=truncate)


reveal_type(truncate)

Output:

scratch.py:14: note: Revealed type is "def (val: Any) -> Any"
Success: no issues found in 1 source file

But this does not:

from collections.abc import Callable
from typing import Any

from attr import define, field


def truncate(length) -> Callable[[Any], Any]:
    def truncate_converter(val):
        return val[:length]

    return truncate_converter


@define
class Person:
    first_name: str = field(converter=truncate(40))
    last_name: str = field(converter=truncate(80))


reveal_type(truncate(40))
reveal_type(truncate(80))

Expected Behavior

Output:

scratch.py:20: note: Revealed type is "def (Any) -> Any"
scratch.py:21: note: Revealed type is "def (Any) -> Any"
Success: no issues found in 1 source file

Actual Behavior

Output:

scratch.py:16: error: Unsupported converter, only named functions, types and lambdas are currently supported  [misc]
scratch.py:17: error: Unsupported converter, only named functions, types and lambdas are currently supported  [misc]
scratch.py:20: note: Revealed type is "def (Any) -> Any"
scratch.py:21: note: Revealed type is "def (Any) -> Any"
Found 2 errors in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 1.4.1
• Mypy command-line flags: just the file name, no extra flags.
• Mypy configuration options from mypy.ini (and other config files):

  [tool.mypy]
  platform = "linux"
  python_version = "3.10"
  

• Python version used: 3.10.12

Start by running the supplied Python reproducer with mypy 1.4.1 and confirm the errors on the two attr converter uses. Trace the attr field converter handling for named functions versus the functions returned by truncate, then add a regression test covering both calls. Done means the higher-order converters are accepted without errors and the revealed callable types remain as shown.