mypy 0.930: regression: no longer keeping contextmanager generic return value

@BarnabyShearer

Bug Report

I bisected this, the regression was introduced in #11352 CC @BarnabyShearer @sobolevn

here's a minimal case split out from pre-commit:

import concurrent.futures
import contextlib
from typing import Callable
from typing import Generator
from typing import Iterable
from typing import TypeVar

TArg = TypeVar('TArg')
TRet = TypeVar('TRet')

@contextlib.contextmanager
def _thread_mapper(maxsize: int) -> Generator[
    Callable[[Callable[[TArg], TRet], Iterable[TArg]], Iterable[TRet]],
    None, None,
]:
    if maxsize == 1:
        yield map
    else:
        with concurrent.futures.ThreadPoolExecutor(maxsize) as ex:
            yield ex.map


def double(x: int) -> int: return x * 2

with _thread_mapper(1) as m:
    print(list(m(double, [2, 3])))

To Reproduce

mypy t.py

Expected Behavior

I expect it to pass (as it did with 0.920)

Actual Behavior

$ mypy ../t.py
../t.py:25: error: Need type annotation for "m"
Found 1 error in 1 file (checked 1 source file)

Your Environment

Mypy version used: 0.930 (regression from 0.920)
Mypy command-line flags: none
Mypy configuration options from mypy.ini (and other config files): none
Python version used: 3.8.10
Operating system and version: ubuntu 20.04

Thanks, I will have a look! 👍

@sobolevn

@sobolevn I think it was a misunderstanding about send type in that PR -- there's no meaning of send types for @contextmanager since it only uses the yield type -- removing detach_callable fixes my snippet above

let me know if you want me to turn this into a patch:

diff --git a/mypy/plugins/default.py b/mypy/plugins/default.py
index 67587090e..c57c5f9a1 100644
--- a/mypy/plugins/default.py
+++ b/mypy/plugins/default.py
@@ -15,7 +15,6 @@ from mypy.types import (
 from mypy.subtypes import is_subtype
 from mypy.typeops import make_simplified_union
 from mypy.checkexpr import is_literal_type_like
-from mypy.checker import detach_callable
 
 
 class DefaultPlugin(Plugin):
@@ -192,12 +191,12 @@ def contextmanager_callback(ctx: FunctionContext) -> Type:
                 and isinstance(default_return, CallableType)):
             # The stub signature doesn't preserve information about arguments so
             # add them back here.
-            return detach_callable(default_return.copy_modified(
+            return default_return.copy_modified(
                 arg_types=arg_type.arg_types,
                 arg_kinds=arg_type.arg_kinds,
                 arg_names=arg_type.arg_names,
                 variables=arg_type.variables,
-                is_ellipsis_args=arg_type.is_ellipsis_args))
+                is_ellipsis_args=arg_type.is_ellipsis_args)
     return ctx.default_return_type
 
 
diff --git a/test-data/unit/check-default-plugin.test b/test-data/unit/check-default-plugin.test
index fc9f132ab..6a89aa266 100644
--- a/test-data/unit/check-default-plugin.test
+++ b/test-data/unit/check-default-plugin.test
@@ -29,10 +29,9 @@ from contextlib import contextmanager
 from typing import TypeVar, Generator
 
 T = TypeVar('T')
-S = TypeVar('S')
 
 @contextmanager
-def yield_id(item: T) -> Generator[T, S, None]:
+def yield_id(item: T) -> Generator[T, None, None]:
     yield item
 
 reveal_type(yield_id) # N: Revealed type is "def [T] (item: T`-1) -> contextlib.GeneratorContextManager[T`-1]"
@@ -70,10 +69,9 @@ from contextlib import asynccontextmanager
 from typing import TypeVar, AsyncGenerator
 
 T = TypeVar('T')
-S = TypeVar('S')
 
 @asynccontextmanager
-async def yield_id(item: T) -> AsyncGenerator[T, S]:
+async def yield_id(item: T) -> AsyncGenerator[T, None]:
     yield item
 
 reveal_type(yield_id) # N: Revealed type is "def [T] (item: T`-1) -> typing.AsyncContextManager[T`-1]"