Fix inference when class and instance match protocol (python#18587)

Fixes python#14688

The bug resulted from (accidentally) inferring against `Iterable` for
both instance and class object. While working on this I noticed there
are also couple flaws in direction handling in constrain inference,
namely:
* A protocol can never ever be a subtype of class object or a `Type[X]`
* When matching against callback protocol, subtype check direction must
match inference direction

I also (conservatively) fix some unrelated issues uncovered by the fix
(to avoid fallout):
* Callable subtyping with trivial suffixes was broken for
positional-only args
* Join of `Parameters` could lead to meaningless results in case of
incompatible arg kinds
* Protocol inference was inconsistent with protocol subtyping w.r.t.
metaclasses.

Author: ilevkivskyi

mypy/constraints.py

@@ -756,40 +756,40 @@ def visit_instance(self, template: Instance) -> list[Constraint]:
                         "__call__", template, actual, is_operator=True
                     )
                     assert call is not None
-                    if mypy.subtypes.is_subtype(actual, erase_typevars(call)):
-                        subres = infer_constraints(call, actual, self.direction)
-                        res.extend(subres)
+                    if (
+                        self.direction == SUPERTYPE_OF
+                        and mypy.subtypes.is_subtype(actual, erase_typevars(call))
+                        or self.direction == SUBTYPE_OF
+                        and mypy.subtypes.is_subtype(erase_typevars(call), actual)
+                    ):
+                        res.extend(infer_constraints(call, actual, self.direction))
                     template.type.inferring.pop()
         if isinstance(actual, CallableType) and actual.fallback is not None:
-            if actual.is_type_obj() and template.type.is_protocol:
+            if (
+                actual.is_type_obj()
+                and template.type.is_protocol
+                and self.direction == SUPERTYPE_OF
+            ):
                 ret_type = get_proper_type(actual.ret_type)
                 if isinstance(ret_type, TupleType):
                     ret_type = mypy.typeops.tuple_fallback(ret_type)
                 if isinstance(ret_type, Instance):
-                    if self.direction == SUBTYPE_OF:
-                        subtype = template
-                    else:
-                        subtype = ret_type
                     res.extend(
                         self.infer_constraints_from_protocol_members(
-                            ret_type, template, subtype, template, class_obj=True
+                            ret_type, template, ret_type, template, class_obj=True
                         )
                     )
             actual = actual.fallback
         if isinstance(actual, TypeType) and template.type.is_protocol:
-            if isinstance(actual.item, Instance):
-                if self.direction == SUBTYPE_OF:
-                    subtype = template
-                else:
-                    subtype = actual.item
-                res.extend(
-                    self.infer_constraints_from_protocol_members(
-                        actual.item, template, subtype, template, class_obj=True
-                    )
-                )
             if self.direction == SUPERTYPE_OF:
-                # Infer constraints for Type[T] via metaclass of T when it makes sense.
                 a_item = actual.item
+                if isinstance(a_item, Instance):
+                    res.extend(
+                        self.infer_constraints_from_protocol_members(
+                            a_item, template, a_item, template, class_obj=True
+                        )
+                    )
+                # Infer constraints for Type[T] via metaclass of T when it makes sense.
                 if isinstance(a_item, TypeVarType):
                     a_item = get_proper_type(a_item.upper_bound)
                 if isinstance(a_item, Instance) and a_item.type.metaclass_type:
@@ -1043,6 +1043,17 @@ def infer_constraints_from_protocol_members(
                 return []  # See #11020
             # The above is safe since at this point we know that 'instance' is a subtype
             # of (erased) 'template', therefore it defines all protocol members
+            if class_obj:
+                # For class objects we must only infer constraints if possible, otherwise it
+                # can lead to confusion between class and instance, for example StrEnum is
+                # Iterable[str] for an instance, but Iterable[StrEnum] for a class object.
+                if not mypy.subtypes.is_subtype(
+                    inst, erase_typevars(temp), ignore_pos_arg_names=True
+                ):
+                    continue
+            # This exception matches the one in subtypes.py, see PR #14121 for context.
+            if member == "__call__" and instance.type.is_metaclass():
+                continue
             res.extend(infer_constraints(temp, inst, self.direction))
             if mypy.subtypes.IS_SETTABLE in mypy.subtypes.get_member_flags(member, protocol):
                 # Settable members are invariant, add opposite constraints

mypy/join.py

@@ -355,7 +355,8 @@ def visit_unpack_type(self, t: UnpackType) -> UnpackType:
 
     def visit_parameters(self, t: Parameters) -> ProperType:
         if isinstance(self.s, Parameters):
-            if len(t.arg_types) != len(self.s.arg_types):
+            if not is_similar_params(t, self.s):
+                # TODO: it would be prudent to return [*object, **object] instead of Any.
                 return self.default(self.s)
             from mypy.meet import meet_types
 
@@ -724,6 +725,15 @@ def is_similar_callables(t: CallableType, s: CallableType) -> bool:
     )
 
 
+def is_similar_params(t: Parameters, s: Parameters) -> bool:
+    # This matches the logic in is_similar_callables() above.
+    return (
+        len(t.arg_types) == len(s.arg_types)
+        and t.min_args == s.min_args
+        and (t.var_arg() is not None) == (s.var_arg() is not None)
+    )
+
+
 def update_callable_ids(c: CallableType, ids: list[TypeVarId]) -> CallableType:
     tv_map = {}
     tvs = []

mypy/subtypes.py

@@ -1719,11 +1719,16 @@ def _incompatible(left_arg: FormalArgument | None, right_arg: FormalArgument | N
         ):
             return False
 
+    if trivial_suffix:
+        # For trivial right suffix we *only* check that every non-star right argument
+        # has a valid match on the left.
+        return True
+
     # Phase 1c: Check var args. Right has an infinite series of optional positional
     #           arguments. Get all further positional args of left, and make sure
     #           they're more general than the corresponding member in right.
     # TODO: are we handling UnpackType correctly here?
-    if right_star is not None and not trivial_suffix:
+    if right_star is not None:
         # Synthesize an anonymous formal argument for the right
         right_by_position = right.try_synthesizing_arg_from_vararg(None)
         assert right_by_position is not None
@@ -1750,7 +1755,7 @@ def _incompatible(left_arg: FormalArgument | None, right_arg: FormalArgument | N
     # Phase 1d: Check kw args. Right has an infinite series of optional named
     #           arguments. Get all further named args of left, and make sure
     #           they're more general than the corresponding member in right.
-    if right_star2 is not None and not trivial_suffix:
+    if right_star2 is not None:
         right_names = {name for name in right.arg_names if name is not None}
         left_only_names = set()
         for name, kind in zip(left.arg_names, left.arg_kinds):

test-data/unit/check-enum.test

@@ -2394,3 +2394,25 @@ def do_check(value: E) -> None:
 
 [builtins fixtures/primitives.pyi]
 [typing fixtures/typing-full.pyi]
+
+[case testStrEnumClassCorrectIterable]
+from enum import StrEnum
+from typing import Type, TypeVar
+
+class Choices(StrEnum):
+    LOREM = "lorem"
+    IPSUM = "ipsum"
+
+var = list(Choices)
+reveal_type(var)  # N: Revealed type is "builtins.list[__main__.Choices]"
+
+e: type[StrEnum]
+reveal_type(list(e))  # N: Revealed type is "builtins.list[enum.StrEnum]"
+
+T = TypeVar("T", bound=StrEnum)
+def list_vals(e: Type[T]) -> list[T]:
+    reveal_type(list(e))  # N: Revealed type is "builtins.list[T`-1]"
+    return list(e)
+
+reveal_type(list_vals(Choices))  # N: Revealed type is "builtins.list[__main__.Choices]"
+[builtins fixtures/enum.pyi]

test-data/unit/check-functions.test

@@ -105,16 +105,38 @@ if int():
     h = h
 
 [case testSubtypingFunctionsDoubleCorrespondence]
+def l(x) -> None: ...
+def r(__x, *, x) -> None: ...
+r = l  # E: Incompatible types in assignment (expression has type "Callable[[Any], None]", variable has type "Callable[[Any, NamedArg(Any, 'x')], None]")
 
+[case testSubtypingFunctionsDoubleCorrespondenceNamedOptional]
 def l(x) -> None: ...
-def r(__, *, x) -> None: ...
-r = l # E: Incompatible types in assignment (expression has type "Callable[[Any], None]", variable has type "Callable[[Any, NamedArg(Any, 'x')], None]")
+def r(__x, *, x = 1) -> None: ...
+r = l  # E: Incompatible types in assignment (expression has type "Callable[[Any], None]", variable has type "Callable[[Any, DefaultNamedArg(Any, 'x')], None]")
 
-[case testSubtypingFunctionsRequiredLeftArgNotPresent]
+[case testSubtypingFunctionsDoubleCorrespondenceBothNamedOptional]
+def l(x = 1) -> None: ...
+def r(__x, *, x = 1) -> None: ...
+r = l  # E: Incompatible types in assignment (expression has type "Callable[[Any], None]", variable has type "Callable[[Any, DefaultNamedArg(Any, 'x')], None]")
+
+[case testSubtypingFunctionsTrivialSuffixRequired]
+def l(__x) -> None: ...
+def r(x, *args, **kwargs) -> None: ...
+
+r = l  # E: Incompatible types in assignment (expression has type "Callable[[Any], None]", variable has type "Callable[[Arg(Any, 'x'), VarArg(Any), KwArg(Any)], None]")
+[builtins fixtures/dict.pyi]
 
+[case testSubtypingFunctionsTrivialSuffixOptional]
+def l(__x = 1) -> None: ...
+def r(x = 1, *args, **kwargs) -> None: ...
+
+r = l  # E: Incompatible types in assignment (expression has type "Callable[[DefaultArg(Any)], None]", variable has type "Callable[[DefaultArg(Any, 'x'), VarArg(Any), KwArg(Any)], None]")
+[builtins fixtures/dict.pyi]
+
+[case testSubtypingFunctionsRequiredLeftArgNotPresent]
 def l(x, y) -> None: ...
 def r(x) -> None: ...
-r = l # E: Incompatible types in assignment (expression has type "Callable[[Any, Any], None]", variable has type "Callable[[Any], None]")
+r = l  # E: Incompatible types in assignment (expression has type "Callable[[Any, Any], None]", variable has type "Callable[[Any], None]")
 
 [case testSubtypingFunctionsImplicitNames]
 from typing import Any

test-data/unit/check-parameter-specification.test

@@ -2532,3 +2532,30 @@ class GenericWrapper(Generic[P]):
         def contains(c: Callable[P, None], *args: P.args, **kwargs: P.kwargs) -> None: ...
         def inherits(*args: P.args, **kwargs: P.kwargs) -> None: ...
 [builtins fixtures/paramspec.pyi]
+
+[case testCallbackProtocolClassObjectParamSpec]
+from typing import Any, Callable, Protocol, Optional, Generic
+from typing_extensions import ParamSpec
+
+P = ParamSpec("P")
+
+class App: ...
+
+class MiddlewareFactory(Protocol[P]):
+    def __call__(self, app: App, /, *args: P.args, **kwargs: P.kwargs) -> App:
+        ...
+
+class Capture(Generic[P]): ...
+
+class ServerErrorMiddleware(App):
+    def __init__(
+        self,
+        app: App,
+        handler: Optional[str] = None,
+        debug: bool = False,
+    ) -> None: ...
+
+def fn(f: MiddlewareFactory[P]) -> Capture[P]: ...
+
+reveal_type(fn(ServerErrorMiddleware))  # N: Revealed type is "__main__.Capture[[handler: Union[builtins.str, None] =, debug: builtins.bool =]]"
+[builtins fixtures/paramspec.pyi]

test-data/unit/fixtures/enum.pyi

@@ -1,5 +1,5 @@
 # Minimal set of builtins required to work with Enums
-from typing import TypeVar, Generic
+from typing import TypeVar, Generic, Iterator, Sequence, overload, Iterable
 
 T = TypeVar('T')
 
@@ -13,6 +13,13 @@ class tuple(Generic[T]):
 class int: pass
 class str:
     def __len__(self) -> int: pass
+    def __iter__(self) -> Iterator[str]: pass
 
 class dict: pass
 class ellipsis: pass
+
+class list(Sequence[T]):
+    @overload
+    def __init__(self) -> None: pass
+    @overload
+    def __init__(self, x: Iterable[T]) -> None: pass