checkmember.py

"""Type checking of attribute access"""

from typing import cast, Callable, Optional, Union, Sequence
from typing_extensions import TYPE_CHECKING

from mypy.types import (
    Type, Instance, AnyType, TupleType, TypedDictType, CallableType, FunctionLike,
    TypeVarLikeType, Overloaded, TypeVarType, UnionType, PartialType, TypeOfAny, LiteralType,
    DeletedType, NoneType, TypeType, has_type_vars, get_proper_type, ProperType, ParamSpecType
)
from mypy.nodes import (
    TypeInfo, FuncBase, Var, FuncDef, SymbolNode, SymbolTable, Context,
    MypyFile, TypeVarExpr, ARG_POS, ARG_STAR, ARG_STAR2, Decorator,
    OverloadedFuncDef, TypeAlias, TempNode, is_final_node,
    SYMBOL_FUNCBASE_TYPES, IndexExpr
)
from mypy.messages import MessageBuilder
from mypy.maptype import map_instance_to_supertype
from mypy.expandtype import expand_type_by_instance, freshen_function_type_vars
from mypy.erasetype import erase_typevars
from mypy.plugin import AttributeContext
from mypy.typeanal import set_any_tvars
from mypy import message_registry
from mypy import subtypes
from mypy import meet
from mypy.typeops import (
    tuple_fallback, bind_self, erase_to_bound, class_callable, type_object_type_from_function,
    make_simplified_union, function_type,
)

if TYPE_CHECKING:  # import for forward declaration only
    import mypy.checker

from mypy import state


class MemberContext:
    """Information and objects needed to type check attribute access.

    Look at the docstring of analyze_member_access for more information.
    """

    def __init__(self,
                 is_lvalue: bool,
                 is_super: bool,
                 is_operator: bool,
                 original_type: Type,
                 context: Context,
                 msg: MessageBuilder,
                 chk: 'mypy.checker.TypeChecker',
                 self_type: Optional[Type],
                 module_symbol_table: Optional[SymbolTable] = None) -> None:
        self.is_lvalue = is_lvalue
        self.is_super = is_super
        self.is_operator = is_operator
        self.original_type = original_type
        self.self_type = self_type or original_type
        self.context = context  # Error context
        self.msg = msg
        self.chk = chk
        self.module_symbol_table = module_symbol_table

    def named_type(self, name: str) -> Instance:
        return self.chk.named_type(name)

    def not_ready_callback(self, name: str, context: Context) -> None:
        self.chk.handle_cannot_determine_type(name, context)

    def copy_modified(self, *, messages: Optional[MessageBuilder] = None,
                      self_type: Optional[Type] = None,
                      is_lvalue: Optional[bool] = None) -> 'MemberContext':
        mx = MemberContext(self.is_lvalue, self.is_super, self.is_operator,
                           self.original_type, self.context, self.msg, self.chk,
                           self.self_type, self.module_symbol_table)
        if messages is not None:
            mx.msg = messages
        if self_type is not None:
            mx.self_type = self_type
        if is_lvalue is not None:
            mx.is_lvalue = is_lvalue
        return mx


def analyze_member_access(name: str,
                          typ: Type,
                          context: Context,
                          is_lvalue: bool,
                          is_super: bool,
                          is_operator: bool,
                          msg: MessageBuilder, *,
                          original_type: Type,
                          chk: 'mypy.checker.TypeChecker',
                          override_info: Optional[TypeInfo] = None,
                          in_literal_context: bool = False,
                          self_type: Optional[Type] = None,
                          module_symbol_table: Optional[SymbolTable] = None) -> Type:
    """Return the type of attribute 'name' of 'typ'.

    The actual implementation is in '_analyze_member_access' and this docstring
    also applies to it.

    This is a general operation that supports various different variations:

      1. lvalue or non-lvalue access (setter or getter access)
      2. supertype access when using super() (is_super == True and
         'override_info' should refer to the supertype)

    'original_type' is the most precise inferred or declared type of the base object
    that we have available. When looking for an attribute of 'typ', we may perform
    recursive calls targeting the fallback type, and 'typ' may become some supertype
    of 'original_type'. 'original_type' is always preserved as the 'typ' type used in
    the initial, non-recursive call. The 'self_type' is a component of 'original_type'
    to which generic self should be bound (a narrower type that has a fallback to instance).
    Currently this is used only for union types.

    'module_symbol_table' is passed to this function if 'typ' is actually a module
    and we want to keep track of the available attributes of the module (since they
    are not available via the type object directly)
    """
    mx = MemberContext(is_lvalue,
                       is_super,
                       is_operator,
                       original_type,
                       context,
                       msg,
                       chk=chk,
                       self_type=self_type,
                       module_symbol_table=module_symbol_table)
    result = _analyze_member_access(name, typ, mx, override_info)
    possible_literal = get_proper_type(result)
    if (in_literal_context and isinstance(possible_literal, Instance) and
            possible_literal.last_known_value is not None):
        return possible_literal.last_known_value
    else:
        return result


def _analyze_member_access(name: str,
                           typ: Type,
                           mx: MemberContext,
                           override_info: Optional[TypeInfo] = None) -> Type:
    # TODO: This and following functions share some logic with subtypes.find_member;
    #       consider refactoring.
    typ = get_proper_type(typ)
    if isinstance(typ, Instance):
        return analyze_instance_member_access(name, typ, mx, override_info)
    elif isinstance(typ, AnyType):
        # The base object has dynamic type.
        return AnyType(TypeOfAny.from_another_any, source_any=typ)
    elif isinstance(typ, UnionType):
        return analyze_union_member_access(name, typ, mx)
    elif isinstance(typ, FunctionLike) and typ.is_type_obj():
        return analyze_type_callable_member_access(name, typ, mx)
    elif isinstance(typ, TypeType):
        return analyze_type_type_member_access(name, typ, mx, override_info)
    elif isinstance(typ, TupleType):
        # Actually look up from the fallback instance type.
        return _analyze_member_access(name, tuple_fallback(typ), mx, override_info)
    elif isinstance(typ, (LiteralType, FunctionLike)):
        # Actually look up from the fallback instance type.
        return _analyze_member_access(name, typ.fallback, mx, override_info)
    elif isinstance(typ, TypedDictType):
        return analyze_typeddict_access(name, typ, mx, override_info)
    elif isinstance(typ, NoneType):
        return analyze_none_member_access(name, typ, mx)
    elif isinstance(typ, TypeVarType):
        return _analyze_member_access(name, typ.upper_bound, mx, override_info)
    elif isinstance(typ, DeletedType):
        mx.msg.deleted_as_rvalue(typ, mx.context)
        return AnyType(TypeOfAny.from_error)
    if mx.chk.should_suppress_optional_error([typ]):
        return AnyType(TypeOfAny.from_error)
    return mx.msg.has_no_attr(mx.original_type, typ, name, mx.context, mx.module_symbol_table)


# The several functions that follow implement analyze_member_access for various
# types and aren't documented individually.


def analyze_instance_member_access(name: str,
                                   typ: Instance,
                                   mx: MemberContext,
                                   override_info: Optional[TypeInfo]) -> Type:
    if name == '__init__' and not mx.is_super:
        # Accessing __init__ in statically typed code would compromise
        # type safety unless used via super().
        mx.msg.fail(message_registry.CANNOT_ACCESS_INIT, mx.context)
        return AnyType(TypeOfAny.from_error)

    # The base object has an instance type.

    info = typ.type
    if override_info:
        info = override_info

    if (state.find_occurrences and
            info.name == state.find_occurrences[0] and
            name == state.find_occurrences[1]):
        mx.msg.note("Occurrence of '{}.{}'".format(*state.find_occurrences), mx.context)

    # Look up the member. First look up the method dictionary.
    method = info.get_method(name)
    if method and not isinstance(method, Decorator):
        if method.is_property:
            assert isinstance(method, OverloadedFuncDef)
            first_item = cast(Decorator, method.items[0])
            return analyze_var(name, first_item.var, typ, info, mx)
        if mx.is_lvalue:
            mx.msg.cant_assign_to_method(mx.context)
        signature = function_type(method, mx.named_type('builtins.function'))
        signature = freshen_function_type_vars(signature)
        if name == '__new__':
            # __new__ is special and behaves like a static method -- don't strip
            # the first argument.
            pass
        else:
            if isinstance(signature, FunctionLike) and name != '__call__':
                # TODO: use proper treatment of special methods on unions instead
                #       of this hack here and below (i.e. mx.self_type).
                dispatched_type = meet.meet_types(mx.original_type, typ)
                signature = check_self_arg(signature, dispatched_type, method.is_class,
                                           mx.context, name, mx.msg)
            signature = bind_self(signature, mx.self_type, is_classmethod=method.is_class)
        typ = map_instance_to_supertype(typ, method.info)
        member_type = expand_type_by_instance(signature, typ)
        freeze_type_vars(member_type)
        return member_type
    else:
        # Not a method.
        return analyze_member_var_access(name, typ, info, mx)


def analyze_type_callable_member_access(name: str,
                                        typ: FunctionLike,
                                        mx: MemberContext) -> Type:
    # Class attribute.
    # TODO super?
    ret_type = typ.items[0].ret_type
    assert isinstance(ret_type, ProperType)
    if isinstance(ret_type, TupleType):
        ret_type = tuple_fallback(ret_type)
    if isinstance(ret_type, Instance):
        if not mx.is_operator:
            # When Python sees an operator (eg `3 == 4`), it automatically translates that
            # into something like `int.__eq__(3, 4)` instead of `(3).__eq__(4)` as an
            # optimization.
            #
            # While it normally it doesn't matter which of the two versions are used, it
            # does cause inconsistencies when working with classes. For example, translating
            # `int == int` to `int.__eq__(int)` would not work since `int.__eq__` is meant to
            # compare two int _instances_. What we really want is `type(int).__eq__`, which
            # is meant to compare two types or classes.
            #
            # This check makes sure that when we encounter an operator, we skip looking up
            # the corresponding method in the current instance to avoid this edge case.
            # See https://github.com/python/mypy/pull/1787 for more info.
            # TODO: do not rely on same type variables being present in all constructor overloads.
            result = analyze_class_attribute_access(ret_type, name, mx,
                                                    original_vars=typ.items[0].variables)
            if result:
                return result
        # Look up from the 'type' type.
        return _analyze_member_access(name, typ.fallback, mx)
    else:
        assert False, 'Unexpected type {}'.format(repr(ret_type))


def analyze_type_type_member_access(name: str,
                                    typ: TypeType,
                                    mx: MemberContext,
                                    override_info: Optional[TypeInfo]) -> Type:
    # Similar to analyze_type_callable_attribute_access.
    item = None
    fallback = mx.named_type('builtins.type')
    ignore_messages = mx.msg.copy()
    ignore_messages.disable_errors().__enter__()
    if isinstance(typ.item, Instance):
        item = typ.item
    elif isinstance(typ.item, AnyType):
        mx = mx.copy_modified(messages=ignore_messages)
        return _analyze_member_access(name, fallback, mx, override_info)
    elif isinstance(typ.item, TypeVarType):
        upper_bound = get_proper_type(typ.item.upper_bound)
        if isinstance(upper_bound, Instance):
            item = upper_bound
        elif isinstance(upper_bound, TupleType):
            item = tuple_fallback(upper_bound)
        elif isinstance(upper_bound, AnyType):
            mx = mx.copy_modified(messages=ignore_messages)
            return _analyze_member_access(name, fallback, mx, override_info)
    elif isinstance(typ.item, TupleType):
        item = tuple_fallback(typ.item)
    elif isinstance(typ.item, FunctionLike) and typ.item.is_type_obj():
        item = typ.item.fallback
    elif isinstance(typ.item, TypeType):
        # Access member on metaclass object via Type[Type[C]]
        if isinstance(typ.item.item, Instance):
            item = typ.item.item.type.metaclass_type
    if item and not mx.is_operator:
        # See comment above for why operators are skipped
        result = analyze_class_attribute_access(item, name, mx, override_info)
        if result:
            if not (isinstance(get_proper_type(result), AnyType) and item.type.fallback_to_any):
                return result
            else:
                # We don't want errors on metaclass lookup for classes with Any fallback
                mx = mx.copy_modified(messages=ignore_messages)
    if item is not None:
        fallback = item.type.metaclass_type or fallback
    return _analyze_member_access(name, fallback, mx, override_info)


def analyze_union_member_access(name: str, typ: UnionType, mx: MemberContext) -> Type:
    mx.msg.disable_type_names += 1
    results = []
    for subtype in typ.relevant_items():
        # Self types should be bound to every individual item of a union.
        item_mx = mx.copy_modified(self_type=subtype)
        results.append(_analyze_member_access(name, subtype, item_mx))
    mx.msg.disable_type_names -= 1
    return make_simplified_union(results)


def analyze_none_member_access(name: str, typ: NoneType, mx: MemberContext) -> Type:
    is_python_3 = mx.chk.options.python_version[0] >= 3
    # In Python 2 "None" has exactly the same attributes as "object". Python 3 adds a single
    # extra attribute, "__bool__".
    if is_python_3 and name == '__bool__':
        literal_false = LiteralType(False, fallback=mx.named_type('builtins.bool'))
        return CallableType(arg_types=[],
                            arg_kinds=[],
                            arg_names=[],
                            ret_type=literal_false,
                            fallback=mx.named_type('builtins.function'))
    elif mx.chk.should_suppress_optional_error([typ]):
        return AnyType(TypeOfAny.from_error)
    else:
        return _analyze_member_access(name, mx.named_type('builtins.object'), mx)


def analyze_member_var_access(name: str,
                              itype: Instance,
                              info: TypeInfo,
                              mx: MemberContext) -> Type:
    """Analyse attribute access that does not target a method.

    This is logically part of analyze_member_access and the arguments are similar.

    original_type is the type of E in the expression E.var
    """
    # It was not a method. Try looking up a variable.
    v = lookup_member_var_or_accessor(info, name, mx.is_lvalue)

    vv = v
    if isinstance(vv, Decorator):
        # The associated Var node of a decorator contains the type.
        v = vv.var

    if isinstance(vv, TypeInfo):
        # If the associated variable is a TypeInfo synthesize a Var node for
        # the purposes of type checking.  This enables us to type check things
        # like accessing class attributes on an inner class.
        v = Var(name, type=type_object_type(vv, mx.named_type))
        v.info = info

    if isinstance(vv, TypeAlias) and isinstance(get_proper_type(vv.target), Instance):
        # Similar to the above TypeInfo case, we allow using
        # qualified type aliases in runtime context if it refers to an
        # instance type. For example:
        #     class C:
        #         A = List[int]
        #     x = C.A() <- this is OK
        typ = instance_alias_type(vv, mx.named_type)
        v = Var(name, type=typ)
        v.info = info

    if isinstance(v, Var):
        implicit = info[name].implicit

        # An assignment to final attribute is always an error,
        # independently of types.
        if mx.is_lvalue and not mx.chk.get_final_context():
            check_final_member(name, info, mx.msg, mx.context)

        return analyze_var(name, v, itype, info, mx, implicit=implicit)
    elif isinstance(v, FuncDef):
        assert False, "Did not expect a function"
    elif (not v and name not in ['__getattr__', '__setattr__', '__getattribute__'] and
          not mx.is_operator and mx.module_symbol_table is None):
        # Above we skip ModuleType.__getattr__ etc. if we have a
        # module symbol table, since the symbol table allows precise
        # checking.
        if not mx.is_lvalue:
            for method_name in ('__getattribute__', '__getattr__'):
                method = info.get_method(method_name)

                # __getattribute__ is defined on builtins.object and returns Any, so without
                # the guard this search will always find object.__getattribute__ and conclude
                # that the attribute exists
                if method and method.info.fullname != 'builtins.object':
                    if isinstance(method, Decorator):
                        # https://github.com/python/mypy/issues/10409
                        bound_method = analyze_var(method_name, method.var, itype, info, mx)
                    else:
                        bound_method = bind_self(
                            function_type(method, mx.named_type('builtins.function')),
                            mx.self_type,
                        )
                    typ = map_instance_to_supertype(itype, method.info)
                    getattr_type = get_proper_type(expand_type_by_instance(bound_method, typ))
                    if isinstance(getattr_type, CallableType):
                        result = getattr_type.ret_type
                    else:
                        result = getattr_type

                    # Call the attribute hook before returning.
                    fullname = '{}.{}'.format(method.info.fullname, name)
                    hook = mx.chk.plugin.get_attribute_hook(fullname)
                    if hook:
                        result = hook(AttributeContext(get_proper_type(mx.original_type),
                                                       result, mx.context, mx.chk))
                    return result
        else:
            setattr_meth = info.get_method('__setattr__')
            if setattr_meth and setattr_meth.info.fullname != 'builtins.object':
                if isinstance(setattr_meth, Decorator):
                    bound_type = analyze_var(
                        name, setattr_meth.var, itype, info,
                        mx.copy_modified(is_lvalue=False),
                    )
                else:
                    bound_type = bind_self(
                        function_type(setattr_meth, mx.named_type('builtins.function')),
                        mx.self_type,
                    )
                typ = map_instance_to_supertype(itype, setattr_meth.info)
                setattr_type = get_proper_type(expand_type_by_instance(bound_type, typ))
                if isinstance(setattr_type, CallableType) and len(setattr_type.arg_types) > 0:
                    return setattr_type.arg_types[-1]

    if itype.type.fallback_to_any:
        return AnyType(TypeOfAny.special_form)

    # Could not find the member.
    if mx.is_super:
        mx.msg.undefined_in_superclass(name, mx.context)
        return AnyType(TypeOfAny.from_error)
    else:
        if mx.chk and mx.chk.should_suppress_optional_error([itype]):
            return AnyType(TypeOfAny.from_error)
        return mx.msg.has_no_attr(
            mx.original_type, itype, name, mx.context, mx.module_symbol_table
        )


def check_final_member(name: str, info: TypeInfo, msg: MessageBuilder, ctx: Context) -> None:
    """Give an error if the name being assigned was declared as final."""
    for base in info.mro:
        sym = base.names.get(name)
        if sym and is_final_node(sym.node):
            msg.cant_assign_to_final(name, attr_assign=True, ctx=ctx)


def analyze_descriptor_access(descriptor_type: Type,
                              mx: MemberContext) -> Type:
    """Type check descriptor access.

    Arguments:
        descriptor_type: The type of the descriptor attribute being accessed
            (the type of ``f`` in ``a.f`` when ``f`` is a descriptor).
        mx: The current member access context.
    Return:
        The return type of the appropriate ``__get__`` overload for the descriptor.
    """
    instance_type = get_proper_type(mx.original_type)
    descriptor_type = get_proper_type(descriptor_type)

    if isinstance(descriptor_type, UnionType):
        # Map the access over union types
        return make_simplified_union([
            analyze_descriptor_access(typ, mx)
            for typ in descriptor_type.items
        ])
    elif not isinstance(descriptor_type, Instance):
        return descriptor_type

    if not descriptor_type.type.has_readable_member('__get__'):
        return descriptor_type

    dunder_get = descriptor_type.type.get_method('__get__')
    if dunder_get is None:
        mx.msg.fail(message_registry.DESCRIPTOR_GET_NOT_CALLABLE.format(descriptor_type),
                    mx.context)
        return AnyType(TypeOfAny.from_error)

    if isinstance(dunder_get, Decorator):
        bound_method = analyze_var(
            '__set__', dunder_get.var, descriptor_type, descriptor_type.type, mx,
        )
    else:
        bound_method = bind_self(
            function_type(dunder_get, mx.named_type('builtins.function')),
            descriptor_type,
        )

    typ = map_instance_to_supertype(descriptor_type, dunder_get.info)
    dunder_get_type = expand_type_by_instance(bound_method, typ)

    if isinstance(instance_type, FunctionLike) and instance_type.is_type_obj():
        owner_type = instance_type.items[0].ret_type
        instance_type = NoneType()
    elif isinstance(instance_type, TypeType):
        owner_type = instance_type.item
        instance_type = NoneType()
    else:
        owner_type = instance_type

    callable_name = mx.chk.expr_checker.method_fullname(descriptor_type, "__get__")
    dunder_get_type = mx.chk.expr_checker.transform_callee_type(
        callable_name, dunder_get_type,
        [TempNode(instance_type, context=mx.context),
         TempNode(TypeType.make_normalized(owner_type), context=mx.context)],
        [ARG_POS, ARG_POS], mx.context, object_type=descriptor_type,
    )

    _, inferred_dunder_get_type = mx.chk.expr_checker.check_call(
        dunder_get_type,
        [TempNode(instance_type, context=mx.context),
         TempNode(TypeType.make_normalized(owner_type), context=mx.context)],
        [ARG_POS, ARG_POS], mx.context, object_type=descriptor_type,
        callable_name=callable_name)

    inferred_dunder_get_type = get_proper_type(inferred_dunder_get_type)
    if isinstance(inferred_dunder_get_type, AnyType):
        # check_call failed, and will have reported an error
        return inferred_dunder_get_type

    if not isinstance(inferred_dunder_get_type, CallableType):
        mx.msg.fail(message_registry.DESCRIPTOR_GET_NOT_CALLABLE.format(descriptor_type),
                    mx.context)
        return AnyType(TypeOfAny.from_error)

    return inferred_dunder_get_type.ret_type


def instance_alias_type(alias: TypeAlias,
                        named_type: Callable[[str], Instance]) -> Type:
    """Type of a type alias node targeting an instance, when appears in runtime context.

    As usual, we first erase any unbound type variables to Any.
    """
    target: Type = get_proper_type(alias.target)
    assert isinstance(get_proper_type(target),
                      Instance), "Must be called only with aliases to classes"
    target = get_proper_type(set_any_tvars(alias, alias.line, alias.column))
    assert isinstance(target, Instance)
    tp = type_object_type(target.type, named_type)
    return expand_type_by_instance(tp, target)


def analyze_var(name: str,
                var: Var,
                itype: Instance,
                info: TypeInfo,
                mx: MemberContext, *,
                implicit: bool = False) -> Type:
    """Analyze access to an attribute via a Var node.

    This is conceptually part of analyze_member_access and the arguments are similar.

    itype is the class object in which var is defined
    original_type is the type of E in the expression E.var
    if implicit is True, the original Var was created as an assignment to self
    """
    # Found a member variable.
    itype = map_instance_to_supertype(itype, var.info)
    typ = var.type
    if typ:
        if isinstance(typ, PartialType):
            return mx.chk.handle_partial_var_type(typ, mx.is_lvalue, var, mx.context)
        if mx.is_lvalue and var.is_property and not var.is_settable_property:
            # TODO allow setting attributes in subclass (although it is probably an error)
            mx.msg.read_only_property(name, itype.type, mx.context)
        if mx.is_lvalue and var.is_classvar:
            mx.msg.cant_assign_to_classvar(name, mx.context)
        t = get_proper_type(expand_type_by_instance(typ, itype))
        result: Type = t
        typ = get_proper_type(typ)
        if var.is_initialized_in_class and isinstance(typ, FunctionLike) and not typ.is_type_obj():
            if mx.is_lvalue:
                if var.is_property:
                    if not var.is_settable_property:
                        mx.msg.read_only_property(name, itype.type, mx.context)
                else:
                    mx.msg.cant_assign_to_method(mx.context)

            if not var.is_staticmethod:
                # Class-level function objects and classmethods become bound methods:
                # the former to the instance, the latter to the class.
                functype = typ
                # Use meet to narrow original_type to the dispatched type.
                # For example, assume
                # * A.f: Callable[[A1], None] where A1 <: A (maybe A1 == A)
                # * B.f: Callable[[B1], None] where B1 <: B (maybe B1 == B)
                # * x: Union[A1, B1]
                # In `x.f`, when checking `x` against A1 we assume x is compatible with A
                # and similarly for B1 when checking against B
                dispatched_type = meet.meet_types(mx.original_type, itype)
                signature = freshen_function_type_vars(functype)
                signature = check_self_arg(signature, dispatched_type, var.is_classmethod,
                                           mx.context, name, mx.msg)
                signature = bind_self(signature, mx.self_type, var.is_classmethod)
                expanded_signature = get_proper_type(expand_type_by_instance(signature, itype))
                freeze_type_vars(expanded_signature)
                if var.is_property:
                    # A property cannot have an overloaded type => the cast is fine.
                    assert isinstance(expanded_signature, CallableType)
                    result = expanded_signature.ret_type
                else:
                    result = expanded_signature
    else:
        if not var.is_ready:
            mx.not_ready_callback(var.name, mx.context)
        # Implicit 'Any' type.
        result = AnyType(TypeOfAny.special_form)
    fullname = '{}.{}'.format(var.info.fullname, name)
    hook = mx.chk.plugin.get_attribute_hook(fullname)
    if result and not mx.is_lvalue and not implicit:
        result = analyze_descriptor_access(result, mx)
    if hook:
        result = hook(AttributeContext(get_proper_type(mx.original_type),
                                       result, mx.context, mx.chk))
    return result


def freeze_type_vars(member_type: Type) -> None:
    if not isinstance(member_type, ProperType):
        return
    if isinstance(member_type, CallableType):
        for v in member_type.variables:
            v.id.meta_level = 0
    if isinstance(member_type, Overloaded):
        for it in member_type.items:
            for v in it.variables:
                v.id.meta_level = 0


def lookup_member_var_or_accessor(info: TypeInfo, name: str,
                                  is_lvalue: bool) -> Optional[SymbolNode]:
    """Find the attribute/accessor node that refers to a member of a type."""
    # TODO handle lvalues
    node = info.get(name)
    if node:
        return node.node
    else:
        return None


def check_self_arg(functype: FunctionLike,
                   dispatched_arg_type: Type,
                   is_classmethod: bool,
                   context: Context, name: str,
                   msg: MessageBuilder) -> FunctionLike:
    """Check that an instance has a valid type for a method with annotated 'self'.

    For example if the method is defined as:
        class A:
            def f(self: S) -> T: ...
    then for 'x.f' we check that meet(type(x), A) <: S. If the method is overloaded, we
    select only overloads items that satisfy this requirement. If there are no matching
    overloads, an error is generated.

    Note: dispatched_arg_type uses a meet to select a relevant item in case if the
    original type of 'x' is a union. This is done because several special methods
    treat union types in ad-hoc manner, so we can't use MemberContext.self_type yet.
    """
    items = functype.items
    if not items:
        return functype
    new_items = []
    if is_classmethod:
        dispatched_arg_type = TypeType.make_normalized(dispatched_arg_type)
    for item in items:
        if not item.arg_types or item.arg_kinds[0] not in (ARG_POS, ARG_STAR):
            # No positional first (self) argument (*args is okay).
            msg.no_formal_self(name, item, context)
            # This is pretty bad, so just return the original signature if
            # there is at least one such error.
            return functype
        else:
            selfarg = item.arg_types[0]
            if subtypes.is_subtype(dispatched_arg_type, erase_typevars(erase_to_bound(selfarg))):
                new_items.append(item)
            elif isinstance(selfarg, ParamSpecType):
                # TODO: This is not always right. What's the most reasonable thing to do here?
                new_items.append(item)
    if not new_items:
        # Choose first item for the message (it may be not very helpful for overloads).
        msg.incompatible_self_argument(name, dispatched_arg_type, items[0],
                                       is_classmethod, context)
        return functype
    if len(new_items) == 1:
        return new_items[0]
    return Overloaded(new_items)


def analyze_class_attribute_access(itype: Instance,
                                   name: str,
                                   mx: MemberContext,
                                   override_info: Optional[TypeInfo] = None,
                                   original_vars: Optional[Sequence[TypeVarLikeType]] = None
                                   ) -> Optional[Type]:
    """Analyze access to an attribute on a class object.

    itype is the return type of the class object callable, original_type is the type
    of E in the expression E.var, original_vars are type variables of the class callable
    (for generic classes).
    """
    info = itype.type
    if override_info:
        info = override_info

    node = info.get(name)
    if not node:
        if info.fallback_to_any:
            return AnyType(TypeOfAny.special_form)
        return None

    is_decorated = isinstance(node.node, Decorator)
    is_method = is_decorated or isinstance(node.node, FuncBase)
    if mx.is_lvalue:
        if is_method:
            mx.msg.cant_assign_to_method(mx.context)
        if isinstance(node.node, TypeInfo):
            mx.msg.fail(message_registry.CANNOT_ASSIGN_TO_TYPE, mx.context)

    # If a final attribute was declared on `self` in `__init__`, then it
    # can't be accessed on the class object.
    if node.implicit and isinstance(node.node, Var) and node.node.is_final:
        mx.msg.fail(message_registry.CANNOT_ACCESS_FINAL_INSTANCE_ATTR
                    .format(node.node.name), mx.context)

    # An assignment to final attribute on class object is also always an error,
    # independently of types.
    if mx.is_lvalue and not mx.chk.get_final_context():
        check_final_member(name, info, mx.msg, mx.context)

    if info.is_enum and not (mx.is_lvalue or is_decorated or is_method):
        enum_class_attribute_type = analyze_enum_class_attribute_access(itype, name, mx)
        if enum_class_attribute_type:
            return enum_class_attribute_type

    t = node.type
    if t:
        if isinstance(t, PartialType):
            symnode = node.node
            assert isinstance(symnode, Var)
            return mx.chk.handle_partial_var_type(t, mx.is_lvalue, symnode, mx.context)

        # Find the class where method/variable was defined.
        if isinstance(node.node, Decorator):
            super_info: Optional[TypeInfo] = node.node.var.info
        elif isinstance(node.node, (Var, SYMBOL_FUNCBASE_TYPES)):
            super_info = node.node.info
        else:
            super_info = None

        # Map the type to how it would look as a defining class. For example:
        #     class C(Generic[T]): ...
        #     class D(C[Tuple[T, S]]): ...
        #     D[int, str].method()
        # Here itype is D[int, str], isuper is C[Tuple[int, str]].
        if not super_info:
            isuper = None
        else:
            isuper = map_instance_to_supertype(itype, super_info)

        if isinstance(node.node, Var):
            assert isuper is not None
            # Check if original variable type has type variables. For example:
            #     class C(Generic[T]):
            #         x: T
            #     C.x  # Error, ambiguous access
            #     C[int].x  # Also an error, since C[int] is same as C at runtime
            if isinstance(t, TypeVarType) or has_type_vars(t):
                # Exception: access on Type[...], including first argument of class methods is OK.
                if not isinstance(get_proper_type(mx.original_type), TypeType) or node.implicit:
                    if node.node.is_classvar:
                        message = message_registry.GENERIC_CLASS_VAR_ACCESS
                    else:
                        message = message_registry.GENERIC_INSTANCE_VAR_CLASS_ACCESS
                    mx.msg.fail(message, mx.context)

            # Erase non-mapped variables, but keep mapped ones, even if there is an error.
            # In the above example this means that we infer following types:
            #     C.x -> Any
            #     C[int].x -> int
            t = erase_typevars(expand_type_by_instance(t, isuper))

        is_classmethod = ((is_decorated and cast(Decorator, node.node).func.is_class)
                          or (isinstance(node.node, FuncBase) and node.node.is_class))
        t = get_proper_type(t)
        if isinstance(t, FunctionLike) and is_classmethod:
            t = check_self_arg(t, mx.self_type, False, mx.context, name, mx.msg)
        result = add_class_tvars(t, isuper, is_classmethod,
                                 mx.self_type, original_vars=original_vars)
        if not mx.is_lvalue:
            result = analyze_descriptor_access(result, mx)
        return result
    elif isinstance(node.node, Var):
        mx.not_ready_callback(name, mx.context)
        return AnyType(TypeOfAny.special_form)

    if isinstance(node.node, TypeVarExpr):
        mx.msg.fail(message_registry.CANNOT_USE_TYPEVAR_AS_EXPRESSION.format(
                    info.name, name), mx.context)
        return AnyType(TypeOfAny.from_error)

    if isinstance(node.node, TypeInfo):
        return type_object_type(node.node, mx.named_type)

    if isinstance(node.node, MypyFile):
        # Reference to a module object.
        return mx.named_type('types.ModuleType')

    if (isinstance(node.node, TypeAlias) and
            isinstance(get_proper_type(node.node.target), Instance)):
        return instance_alias_type(node.node, mx.named_type)

    if is_decorated:
        assert isinstance(node.node, Decorator)
        if node.node.type:
            return node.node.type
        else:
            mx.not_ready_callback(name, mx.context)
            return AnyType(TypeOfAny.from_error)
    else:
        assert isinstance(node.node, FuncBase)
        typ = function_type(node.node, mx.named_type('builtins.function'))
        # Note: if we are accessing class method on class object, the cls argument is bound.
        # Annotated and/or explicit class methods go through other code paths above, for
        # unannotated implicit class methods we do this here.
        if node.node.is_class:
            typ = bind_self(typ, is_classmethod=True)
        return typ


def analyze_enum_class_attribute_access(itype: Instance,
                                        name: str,
                                        mx: MemberContext,
                                        ) -> Optional[Type]:
    # Skip "_order_" and "__order__", since Enum will remove it
    if name in ("_order_", "__order__"):
        return mx.msg.has_no_attr(
            mx.original_type, itype, name, mx.context, mx.module_symbol_table
        )
    # For other names surrendered by underscores, we don't make them Enum members
    if name.startswith('__') and name.endswith("__") and name.replace('_', '') != '':
        return None

    enum_literal = LiteralType(name, fallback=itype)
    # When we analyze enums, the corresponding Instance is always considered to be erased
    # due to how the signature of Enum.__new__ is `(cls: Type[_T], value: object) -> _T`
    # in typeshed. However, this is really more of an implementation detail of how Enums
    # are typed, and we really don't want to treat every single Enum value as if it were
    # from type variable substitution. So we reset the 'erased' field here.
    return itype.copy_modified(erased=False, last_known_value=enum_literal)


def analyze_typeddict_access(name: str, typ: TypedDictType,
                             mx: MemberContext, override_info: Optional[TypeInfo]) -> Type:
    if name == '__setitem__':
        if isinstance(mx.context, IndexExpr):
            # Since we can get this during `a['key'] = ...`
            # it is safe to assume that the context is `IndexExpr`.
            item_type = mx.chk.expr_checker.visit_typeddict_index_expr(
                typ, mx.context.index)
        else:
            # It can also be `a.__setitem__(...)` direct call.
            # In this case `item_type` can be `Any`,
            # because we don't have args available yet.
            # TODO: check in `default` plugin that `__setitem__` is correct.
            item_type = AnyType(TypeOfAny.implementation_artifact)
        return CallableType(
            arg_types=[mx.chk.named_type('builtins.str'), item_type],
            arg_kinds=[ARG_POS, ARG_POS],
            arg_names=[None, None],
            ret_type=NoneType(),
            fallback=mx.chk.named_type('builtins.function'),
            name=name,
        )
    elif name == '__delitem__':
        return CallableType(
            arg_types=[mx.chk.named_type('builtins.str')],
            arg_kinds=[ARG_POS],
            arg_names=[None],
            ret_type=NoneType(),
            fallback=mx.chk.named_type('builtins.function'),
            name=name,
        )
    return _analyze_member_access(name, typ.fallback, mx, override_info)


def add_class_tvars(t: ProperType, isuper: Optional[Instance],
                    is_classmethod: bool,
                    original_type: Type,
                    original_vars: Optional[Sequence[TypeVarLikeType]] = None) -> Type:
    """Instantiate type variables during analyze_class_attribute_access,
    e.g T and Q in the following:

    class A(Generic[T]):
        @classmethod
        def foo(cls: Type[Q]) -> Tuple[T, Q]: ...

    class B(A[str]): pass
    B.foo()

    Args:
        t: Declared type of the method (or property)
        isuper: Current instance mapped to the superclass where method was defined, this
            is usually done by map_instance_to_supertype()
        is_classmethod: True if this method is decorated with @classmethod
        original_type: The value of the type B in the expression B.foo() or the corresponding
            component in case of a union (this is used to bind the self-types)
        original_vars: Type variables of the class callable on which the method was accessed
    Returns:
        Expanded method type with added type variables (when needed).
    """
    # TODO: verify consistency between Q and T

    # We add class type variables if the class method is accessed on class object
    # without applied type arguments, this matches the behavior of __init__().
    # For example (continuing the example in docstring):
    #     A       # The type of callable is def [T] () -> A[T], _not_ def () -> A[Any]
    #     A[int]  # The type of callable is def () -> A[int]
    # and
    #     A.foo       # The type is generic def [T] () -> Tuple[T, A[T]]
    #     A[int].foo  # The type is non-generic def () -> Tuple[int, A[int]]
    #
    # This behaviour is useful for defining alternative constructors for generic classes.
    # To achieve such behaviour, we add the class type variables that are still free
    # (i.e. appear in the return type of the class object on which the method was accessed).
    if isinstance(t, CallableType):
        tvars = original_vars if original_vars is not None else []
        if is_classmethod:
            t = freshen_function_type_vars(t)
            t = bind_self(t, original_type, is_classmethod=True)
            assert isuper is not None
            t = cast(CallableType, expand_type_by_instance(t, isuper))
            freeze_type_vars(t)
        return t.copy_modified(variables=list(tvars) + list(t.variables))
    elif isinstance(t, Overloaded):
        return Overloaded([cast(CallableType, add_class_tvars(item, isuper,
                                                              is_classmethod, original_type,
                                                              original_vars=original_vars))
                           for item in t.items])
    if isuper is not None:
        t = cast(ProperType, expand_type_by_instance(t, isuper))
    return t


def type_object_type(info: TypeInfo, named_type: Callable[[str], Instance]) -> ProperType:
    """Return the type of a type object.

    For a generic type G with type variables T and S the type is generally of form

      Callable[..., G[T, S]]

    where ... are argument types for the __init__/__new__ method (without the self
    argument). Also, the fallback type will be 'type' instead of 'function'.
    """

    # We take the type from whichever of __init__ and __new__ is first
    # in the MRO, preferring __init__ if there is a tie.
    init_method = info.get('__init__')
    new_method = info.get('__new__')
    if not init_method or not is_valid_constructor(init_method.node):
        # Must be an invalid class definition.
        return AnyType(TypeOfAny.from_error)
    # There *should* always be a __new__ method except the test stubs
    # lack it, so just copy init_method in that situation
    new_method = new_method or init_method
    if not is_valid_constructor(new_method.node):
        # Must be an invalid class definition.
        return AnyType(TypeOfAny.from_error)

    # The two is_valid_constructor() checks ensure this.
    assert isinstance(new_method.node, (SYMBOL_FUNCBASE_TYPES, Decorator))
    assert isinstance(init_method.node, (SYMBOL_FUNCBASE_TYPES, Decorator))

    init_index = info.mro.index(init_method.node.info)
    new_index = info.mro.index(new_method.node.info)

    fallback = info.metaclass_type or named_type('builtins.type')
    if init_index < new_index:
        method: Union[FuncBase, Decorator] = init_method.node
        is_new = False
    elif init_index > new_index:
        method = new_method.node
        is_new = True
    else:
        if init_method.node.info.fullname == 'builtins.object':
            # Both are defined by object.  But if we've got a bogus
            # base class, we can't know for sure, so check for that.
            if info.fallback_to_any:
                # Construct a universal callable as the prototype.
                any_type = AnyType(TypeOfAny.special_form)
                sig = CallableType(arg_types=[any_type, any_type],
                                   arg_kinds=[ARG_STAR, ARG_STAR2],
                                   arg_names=["_args", "_kwds"],
                                   ret_type=any_type,
                                   fallback=named_type('builtins.function'))
                return class_callable(sig, info, fallback, None, is_new=False)

        # Otherwise prefer __init__ in a tie. It isn't clear that this
        # is the right thing, but __new__ caused problems with
        # typeshed (#5647).
        method = init_method.node
        is_new = False
    # Construct callable type based on signature of __init__. Adjust
    # return type and insert type arguments.
    if isinstance(method, FuncBase):
        t = function_type(method, fallback)
    else:
        assert isinstance(method.type, ProperType)
        assert isinstance(method.type, FunctionLike)  # is_valid_constructor() ensures this
        t = method.type
    return type_object_type_from_function(t, info, method.info, fallback, is_new)


def is_valid_constructor(n: Optional[SymbolNode]) -> bool:
    """Does this node represents a valid constructor method?

    This includes normal functions, overloaded functions, and decorators
    that return a callable type.
    """
    if isinstance(n, FuncBase):
        return True
    if isinstance(n, Decorator):
        return isinstance(get_proper_type(n.type), FunctionLike)
    return False