False positive error when subclassing List type

[thepabloaguilar]

Bug Report

This bug is experienced in classes lib

To Reproduce

from typing import List


class ListOfStrMeta(type):
    def __instancecheck__(cls, other) -> bool:
        return (
            isinstance(other, list) and
            bool(other) and
            all(isinstance(list_item, str) for list_item in other)
        )


class ListOfStr(List[str], metaclass=ListOfStrMeta):
    ...

Expected Behavior

No errors should appear

Actual Behavior

Error from the code above:

poetry run mypy ex.py
ex.py:13: error: Metaclass conflict: the metaclass of a derived class must be a (non-strict) subclass of the metaclasses of all its bases  [misc]
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 1.2.0
• Mypy command-line flags: N/A
• Mypy configuration options from mypy.ini (and other config files):

[mypy]
# Plugins, includes custom:
plugins =
  classes.contrib.mypy.classes_plugin

disable_error_code = empty-body

allow_redefinition = false
check_untyped_defs = true
disallow_any_explicit = true
# disallow_any_generics = true
disallow_untyped_calls = true
ignore_errors = false
ignore_missing_imports = true
implicit_reexport = false
local_partial_types = true
strict_optional = true
strict_equality = true
no_implicit_optional = true
warn_no_return = true
warn_unused_ignores = true
warn_redundant_casts = true
warn_unused_configs = true
warn_unreachable = true

• Python version used: 3.10.10

cc @sobolevn

[ikonst]

This is probably due to #6042:

mypy/mypy/semanal.py

Lines 2363 to 2372 in 0f09be4

	if any(info.is_protocol for info in defn.info.mro):
	if (
	not defn.info.metaclass_type
	or defn.info.metaclass_type.type.fullname == "builtins.type"
	):
	# All protocols and their subclasses have ABCMeta metaclass by default.
	# TODO: add a metaclass conflict check if there is another metaclass.
	abc_meta = self.named_type_or_none("abc.ABCMeta", [])
	if abc_meta is not None: # May be None in tests with incomplete lib-stub.
	defn.info.metaclass_type = abc_meta

Can take a look @ilevkivskyi?

Since list derives from a handful of protocols (e.g. Sequence), its metaclass is ABCMeta.

Changing

-class ListOfStrMeta(typing):
+class ListOfStrMeta(ABCMeta):

eliminates the error, though it's probably wrong since type(list) is type.

[sobolevn]

Yes, this is due to the fact that typeshed has Sequence in the base classes of list.
While runtime does not have it.