Remove contextlib plugin from default (#13923)

It is no longer needed. `typeshed` defines `contextlib.contextmanager`
and `contextlib.asynccontextmanager` using `ParamSpec`, which works the
same way as this plugin.

mypy/plugins/default.py

@@ -39,9 +39,7 @@ class DefaultPlugin(Plugin):
     def get_function_hook(self, fullname: str) -> Callable[[FunctionContext], Type] | None:
         from mypy.plugins import ctypes, singledispatch
 
-        if fullname in ("contextlib.contextmanager", "contextlib.asynccontextmanager"):
-            return contextmanager_callback
-        elif fullname == "ctypes.Array":
+        if fullname == "ctypes.Array":
             return ctypes.array_constructor_callback
         elif fullname == "functools.singledispatch":
             return singledispatch.create_singledispatch_function_callback
@@ -148,25 +146,6 @@ def get_class_decorator_hook_2(
         return None
 
 
-def contextmanager_callback(ctx: FunctionContext) -> Type:
-    """Infer a better return type for 'contextlib.contextmanager'."""
-    # Be defensive, just in case.
-    if ctx.arg_types and len(ctx.arg_types[0]) == 1:
-        arg_type = get_proper_type(ctx.arg_types[0][0])
-        default_return = get_proper_type(ctx.default_return_type)
-        if isinstance(arg_type, CallableType) and isinstance(default_return, CallableType):
-            # The stub signature doesn't preserve information about arguments so
-            # add them back here.
-            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,
-            )
-    return ctx.default_return_type
-
-
 def typed_dict_get_signature_callback(ctx: MethodSigContext) -> CallableType:
     """Try to infer a better signature type for TypedDict.get.
 

test-data/unit/lib-stub/contextlib.pyi

@@ -7,6 +7,7 @@ _T = TypeVar('_T')
 class GeneratorContextManager(ContextManager[_T], Generic[_T]):
     def __call__(self, func: Callable[..., _T]) -> Callable[..., _T]: ...
 
+# This does not match `typeshed` definition, needs `ParamSpec`:
 def contextmanager(func: Callable[..., Iterator[_T]]) -> Callable[..., GeneratorContextManager[_T]]:
     ...