Major fixes and new features

venv/lib/python3.12/site-packages/mypy/checkpattern.py

@@ -0,0 +1,721 @@
+"""Pattern checker. This file is conceptually part of TypeChecker."""
+
+from __future__ import annotations
+
+from collections import defaultdict
+from typing import Final, NamedTuple
+
+import mypy.checker
+from mypy import message_registry
+from mypy.checkmember import analyze_member_access
+from mypy.expandtype import expand_type_by_instance
+from mypy.join import join_types
+from mypy.literals import literal_hash
+from mypy.maptype import map_instance_to_supertype
+from mypy.meet import narrow_declared_type
+from mypy.messages import MessageBuilder
+from mypy.nodes import ARG_POS, Context, Expression, NameExpr, TypeAlias, TypeInfo, Var
+from mypy.options import Options
+from mypy.patterns import (
+    AsPattern,
+    ClassPattern,
+    MappingPattern,
+    OrPattern,
+    Pattern,
+    SequencePattern,
+    SingletonPattern,
+    StarredPattern,
+    ValuePattern,
+)
+from mypy.plugin import Plugin
+from mypy.subtypes import is_subtype
+from mypy.typeops import (
+    coerce_to_literal,
+    make_simplified_union,
+    try_getting_str_literals_from_type,
+    tuple_fallback,
+)
+from mypy.types import (
+    AnyType,
+    Instance,
+    LiteralType,
+    NoneType,
+    ProperType,
+    TupleType,
+    Type,
+    TypedDictType,
+    TypeOfAny,
+    UninhabitedType,
+    UnionType,
+    get_proper_type,
+)
+from mypy.typevars import fill_typevars
+from mypy.visitor import PatternVisitor
+
+self_match_type_names: Final = [
+    "builtins.bool",
+    "builtins.bytearray",
+    "builtins.bytes",
+    "builtins.dict",
+    "builtins.float",
+    "builtins.frozenset",
+    "builtins.int",
+    "builtins.list",
+    "builtins.set",
+    "builtins.str",
+    "builtins.tuple",
+]
+
+non_sequence_match_type_names: Final = ["builtins.str", "builtins.bytes", "builtins.bytearray"]
+
+
+# For every Pattern a PatternType can be calculated. This requires recursively calculating
+# the PatternTypes of the sub-patterns first.
+# Using the data in the PatternType the match subject and captured names can be narrowed/inferred.
+class PatternType(NamedTuple):
+    type: Type  # The type the match subject can be narrowed to
+    rest_type: Type  # The remaining type if the pattern didn't match
+    captures: dict[Expression, Type]  # The variables captured by the pattern
+
+
+class PatternChecker(PatternVisitor[PatternType]):
+    """Pattern checker.
+
+    This class checks if a pattern can match a type, what the type can be narrowed to, and what
+    type capture patterns should be inferred as.
+    """
+
+    # Some services are provided by a TypeChecker instance.
+    chk: mypy.checker.TypeChecker
+    # This is shared with TypeChecker, but stored also here for convenience.
+    msg: MessageBuilder
+    # Currently unused
+    plugin: Plugin
+    # The expression being matched against the pattern
+    subject: Expression
+
+    subject_type: Type
+    # Type of the subject to check the (sub)pattern against
+    type_context: list[Type]
+    # Types that match against self instead of their __match_args__ if used as a class pattern
+    # Filled in from self_match_type_names
+    self_match_types: list[Type]
+    # Types that are sequences, but don't match sequence patterns. Filled in from
+    # non_sequence_match_type_names
+    non_sequence_match_types: list[Type]
+
+    options: Options
+
+    def __init__(
+        self, chk: mypy.checker.TypeChecker, msg: MessageBuilder, plugin: Plugin, options: Options
+    ) -> None:
+        self.chk = chk
+        self.msg = msg
+        self.plugin = plugin
+
+        self.type_context = []
+        self.self_match_types = self.generate_types_from_names(self_match_type_names)
+        self.non_sequence_match_types = self.generate_types_from_names(
+            non_sequence_match_type_names
+        )
+        self.options = options
+
+    def accept(self, o: Pattern, type_context: Type) -> PatternType:
+        self.type_context.append(type_context)
+        result = o.accept(self)
+        self.type_context.pop()
+
+        return result
+
+    def visit_as_pattern(self, o: AsPattern) -> PatternType:
+        current_type = self.type_context[-1]
+        if o.pattern is not None:
+            pattern_type = self.accept(o.pattern, current_type)
+            typ, rest_type, type_map = pattern_type
+        else:
+            typ, rest_type, type_map = current_type, UninhabitedType(), {}
+
+        if not is_uninhabited(typ) and o.name is not None:
+            typ, _ = self.chk.conditional_types_with_intersection(
+                current_type, [get_type_range(typ)], o, default=current_type
+            )
+            if not is_uninhabited(typ):
+                type_map[o.name] = typ
+
+        return PatternType(typ, rest_type, type_map)
+
+    def visit_or_pattern(self, o: OrPattern) -> PatternType:
+        current_type = self.type_context[-1]
+
+        #
+        # Check all the subpatterns
+        #
+        pattern_types = []
+        for pattern in o.patterns:
+            pattern_type = self.accept(pattern, current_type)
+            pattern_types.append(pattern_type)
+            current_type = pattern_type.rest_type
+
+        #
+        # Collect the final type
+        #
+        types = []
+        for pattern_type in pattern_types:
+            if not is_uninhabited(pattern_type.type):
+                types.append(pattern_type.type)
+
+        #
+        # Check the capture types
+        #
+        capture_types: dict[Var, list[tuple[Expression, Type]]] = defaultdict(list)
+        # Collect captures from the first subpattern
+        for expr, typ in pattern_types[0].captures.items():
+            node = get_var(expr)
+            capture_types[node].append((expr, typ))
+
+        # Check if other subpatterns capture the same names
+        for i, pattern_type in enumerate(pattern_types[1:]):
+            vars = {get_var(expr) for expr, _ in pattern_type.captures.items()}
+            if capture_types.keys() != vars:
+                self.msg.fail(message_registry.OR_PATTERN_ALTERNATIVE_NAMES, o.patterns[i])
+            for expr, typ in pattern_type.captures.items():
+                node = get_var(expr)
+                capture_types[node].append((expr, typ))
+
+        captures: dict[Expression, Type] = {}
+        for var, capture_list in capture_types.items():
+            typ = UninhabitedType()
+            for _, other in capture_list:
+                typ = join_types(typ, other)
+
+            captures[capture_list[0][0]] = typ
+
+        union_type = make_simplified_union(types)
+        return PatternType(union_type, current_type, captures)
+
+    def visit_value_pattern(self, o: ValuePattern) -> PatternType:
+        current_type = self.type_context[-1]
+        typ = self.chk.expr_checker.accept(o.expr)
+        typ = coerce_to_literal(typ)
+        narrowed_type, rest_type = self.chk.conditional_types_with_intersection(
+            current_type, [get_type_range(typ)], o, default=current_type
+        )
+        if not isinstance(get_proper_type(narrowed_type), (LiteralType, UninhabitedType)):
+            return PatternType(narrowed_type, UnionType.make_union([narrowed_type, rest_type]), {})
+        return PatternType(narrowed_type, rest_type, {})
+
+    def visit_singleton_pattern(self, o: SingletonPattern) -> PatternType:
+        current_type = self.type_context[-1]
+        value: bool | None = o.value
+        if isinstance(value, bool):
+            typ = self.chk.expr_checker.infer_literal_expr_type(value, "builtins.bool")
+        elif value is None:
+            typ = NoneType()
+        else:
+            assert False
+
+        narrowed_type, rest_type = self.chk.conditional_types_with_intersection(
+            current_type, [get_type_range(typ)], o, default=current_type
+        )
+        return PatternType(narrowed_type, rest_type, {})
+
+    def visit_sequence_pattern(self, o: SequencePattern) -> PatternType:
+        #
+        # check for existence of a starred pattern
+        #
+        current_type = get_proper_type(self.type_context[-1])
+        if not self.can_match_sequence(current_type):
+            return self.early_non_match()
+        star_positions = [i for i, p in enumerate(o.patterns) if isinstance(p, StarredPattern)]
+        star_position: int | None = None
+        if len(star_positions) == 1:
+            star_position = star_positions[0]
+        elif len(star_positions) >= 2:
+            assert False, "Parser should prevent multiple starred patterns"
+        required_patterns = len(o.patterns)
+        if star_position is not None:
+            required_patterns -= 1
+
+        #
+        # get inner types of original type
+        #
+        if isinstance(current_type, TupleType):
+            inner_types = current_type.items
+            size_diff = len(inner_types) - required_patterns
+            if size_diff < 0:
+                return self.early_non_match()
+            elif size_diff > 0 and star_position is None:
+                return self.early_non_match()
+        else:
+            inner_type = self.get_sequence_type(current_type, o)
+            if inner_type is None:
+                inner_type = self.chk.named_type("builtins.object")
+            inner_types = [inner_type] * len(o.patterns)
+
+        #
+        # match inner patterns
+        #
+        contracted_new_inner_types: list[Type] = []
+        contracted_rest_inner_types: list[Type] = []
+        captures: dict[Expression, Type] = {}
+
+        contracted_inner_types = self.contract_starred_pattern_types(
+            inner_types, star_position, required_patterns
+        )
+        for p, t in zip(o.patterns, contracted_inner_types):
+            pattern_type = self.accept(p, t)
+            typ, rest, type_map = pattern_type
+            contracted_new_inner_types.append(typ)
+            contracted_rest_inner_types.append(rest)
+            self.update_type_map(captures, type_map)
+
+        new_inner_types = self.expand_starred_pattern_types(
+            contracted_new_inner_types, star_position, len(inner_types)
+        )
+        rest_inner_types = self.expand_starred_pattern_types(
+            contracted_rest_inner_types, star_position, len(inner_types)
+        )
+
+        #
+        # Calculate new type
+        #
+        new_type: Type
+        rest_type: Type = current_type
+        if isinstance(current_type, TupleType):
+            narrowed_inner_types = []
+            inner_rest_types = []
+            for inner_type, new_inner_type in zip(inner_types, new_inner_types):
+                (
+                    narrowed_inner_type,
+                    inner_rest_type,
+                ) = self.chk.conditional_types_with_intersection(
+                    new_inner_type, [get_type_range(inner_type)], o, default=new_inner_type
+                )
+                narrowed_inner_types.append(narrowed_inner_type)
+                inner_rest_types.append(inner_rest_type)
+            if all(not is_uninhabited(typ) for typ in narrowed_inner_types):
+                new_type = TupleType(narrowed_inner_types, current_type.partial_fallback)
+            else:
+                new_type = UninhabitedType()
+
+            if all(is_uninhabited(typ) for typ in inner_rest_types):
+                # All subpatterns always match, so we can apply negative narrowing
+                rest_type = TupleType(rest_inner_types, current_type.partial_fallback)
+        else:
+            new_inner_type = UninhabitedType()
+            for typ in new_inner_types:
+                new_inner_type = join_types(new_inner_type, typ)
+            new_type = self.construct_sequence_child(current_type, new_inner_type)
+            if is_subtype(new_type, current_type):
+                new_type, _ = self.chk.conditional_types_with_intersection(
+                    current_type, [get_type_range(new_type)], o, default=current_type
+                )
+            else:
+                new_type = current_type
+        return PatternType(new_type, rest_type, captures)
+
+    def get_sequence_type(self, t: Type, context: Context) -> Type | None:
+        t = get_proper_type(t)
+        if isinstance(t, AnyType):
+            return AnyType(TypeOfAny.from_another_any, t)
+        if isinstance(t, UnionType):
+            items = [self.get_sequence_type(item, context) for item in t.items]
+            not_none_items = [item for item in items if item is not None]
+            if not_none_items:
+                return make_simplified_union(not_none_items)
+            else:
+                return None
+
+        if self.chk.type_is_iterable(t) and isinstance(t, (Instance, TupleType)):
+            if isinstance(t, TupleType):
+                t = tuple_fallback(t)
+            return self.chk.iterable_item_type(t, context)
+        else:
+            return None
+
+    def contract_starred_pattern_types(
+        self, types: list[Type], star_pos: int | None, num_patterns: int
+    ) -> list[Type]:
+        """
+        Contracts a list of types in a sequence pattern depending on the position of a starred
+        capture pattern.
+
+        For example if the sequence pattern [a, *b, c] is matched against types [bool, int, str,
+        bytes] the contracted types are [bool, Union[int, str], bytes].
+
+        If star_pos in None the types are returned unchanged.
+        """
+        if star_pos is None:
+            return types
+        new_types = types[:star_pos]
+        star_length = len(types) - num_patterns
+        new_types.append(make_simplified_union(types[star_pos : star_pos + star_length]))
+        new_types += types[star_pos + star_length :]
+
+        return new_types
+
+    def expand_starred_pattern_types(
+        self, types: list[Type], star_pos: int | None, num_types: int
+    ) -> list[Type]:
+        """Undoes the contraction done by contract_starred_pattern_types.
+
+        For example if the sequence pattern is [a, *b, c] and types [bool, int, str] are extended
+        to length 4 the result is [bool, int, int, str].
+        """
+        if star_pos is None:
+            return types
+        new_types = types[:star_pos]
+        star_length = num_types - len(types) + 1
+        new_types += [types[star_pos]] * star_length
+        new_types += types[star_pos + 1 :]
+
+        return new_types
+
+    def visit_starred_pattern(self, o: StarredPattern) -> PatternType:
+        captures: dict[Expression, Type] = {}
+        if o.capture is not None:
+            list_type = self.chk.named_generic_type("builtins.list", [self.type_context[-1]])
+            captures[o.capture] = list_type
+        return PatternType(self.type_context[-1], UninhabitedType(), captures)
+
+    def visit_mapping_pattern(self, o: MappingPattern) -> PatternType:
+        current_type = get_proper_type(self.type_context[-1])
+        can_match = True
+        captures: dict[Expression, Type] = {}
+        for key, value in zip(o.keys, o.values):
+            inner_type = self.get_mapping_item_type(o, current_type, key)
+            if inner_type is None:
+                can_match = False
+                inner_type = self.chk.named_type("builtins.object")
+            pattern_type = self.accept(value, inner_type)
+            if is_uninhabited(pattern_type.type):
+                can_match = False
+            else:
+                self.update_type_map(captures, pattern_type.captures)
+
+        if o.rest is not None:
+            mapping = self.chk.named_type("typing.Mapping")
+            if is_subtype(current_type, mapping) and isinstance(current_type, Instance):
+                mapping_inst = map_instance_to_supertype(current_type, mapping.type)
+                dict_typeinfo = self.chk.lookup_typeinfo("builtins.dict")
+                rest_type = Instance(dict_typeinfo, mapping_inst.args)
+            else:
+                object_type = self.chk.named_type("builtins.object")
+                rest_type = self.chk.named_generic_type(
+                    "builtins.dict", [object_type, object_type]
+                )
+
+            captures[o.rest] = rest_type
+
+        if can_match:
+            # We can't narrow the type here, as Mapping key is invariant.
+            new_type = self.type_context[-1]
+        else:
+            new_type = UninhabitedType()
+        return PatternType(new_type, current_type, captures)
+
+    def get_mapping_item_type(
+        self, pattern: MappingPattern, mapping_type: Type, key: Expression
+    ) -> Type | None:
+        mapping_type = get_proper_type(mapping_type)
+        if isinstance(mapping_type, TypedDictType):
+            with self.msg.filter_errors() as local_errors:
+                result: Type | None = self.chk.expr_checker.visit_typeddict_index_expr(
+                    mapping_type, key
+                )
+                has_local_errors = local_errors.has_new_errors()
+            # If we can't determine the type statically fall back to treating it as a normal
+            # mapping
+            if has_local_errors:
+                with self.msg.filter_errors() as local_errors:
+                    result = self.get_simple_mapping_item_type(pattern, mapping_type, key)
+
+                    if local_errors.has_new_errors():
+                        result = None
+        else:
+            with self.msg.filter_errors():
+                result = self.get_simple_mapping_item_type(pattern, mapping_type, key)
+        return result
+
+    def get_simple_mapping_item_type(
+        self, pattern: MappingPattern, mapping_type: Type, key: Expression
+    ) -> Type:
+        result, _ = self.chk.expr_checker.check_method_call_by_name(
+            "__getitem__", mapping_type, [key], [ARG_POS], pattern
+        )
+        return result
+
+    def visit_class_pattern(self, o: ClassPattern) -> PatternType:
+        current_type = get_proper_type(self.type_context[-1])
+
+        #
+        # Check class type
+        #
+        type_info = o.class_ref.node
+        if type_info is None:
+            return PatternType(AnyType(TypeOfAny.from_error), AnyType(TypeOfAny.from_error), {})
+        if isinstance(type_info, TypeAlias) and not type_info.no_args:
+            self.msg.fail(message_registry.CLASS_PATTERN_GENERIC_TYPE_ALIAS, o)
+            return self.early_non_match()
+        if isinstance(type_info, TypeInfo):
+            any_type = AnyType(TypeOfAny.implementation_artifact)
+            typ: Type = Instance(type_info, [any_type] * len(type_info.defn.type_vars))
+        elif isinstance(type_info, TypeAlias):
+            typ = type_info.target
+        else:
+            if isinstance(type_info, Var) and type_info.type is not None:
+                name = type_info.type.str_with_options(self.options)
+            else:
+                name = type_info.name
+            self.msg.fail(message_registry.CLASS_PATTERN_TYPE_REQUIRED.format(name), o)
+            return self.early_non_match()
+
+        new_type, rest_type = self.chk.conditional_types_with_intersection(
+            current_type, [get_type_range(typ)], o, default=current_type
+        )
+        if is_uninhabited(new_type):
+            return self.early_non_match()
+        # TODO: Do I need this?
+        narrowed_type = narrow_declared_type(current_type, new_type)
+
+        #
+        # Convert positional to keyword patterns
+        #
+        keyword_pairs: list[tuple[str | None, Pattern]] = []
+        match_arg_set: set[str] = set()
+
+        captures: dict[Expression, Type] = {}
+
+        if len(o.positionals) != 0:
+            if self.should_self_match(typ):
+                if len(o.positionals) > 1:
+                    self.msg.fail(message_registry.CLASS_PATTERN_TOO_MANY_POSITIONAL_ARGS, o)
+                pattern_type = self.accept(o.positionals[0], narrowed_type)
+                if not is_uninhabited(pattern_type.type):
+                    return PatternType(
+                        pattern_type.type,
+                        join_types(rest_type, pattern_type.rest_type),
+                        pattern_type.captures,
+                    )
+                captures = pattern_type.captures
+            else:
+                with self.msg.filter_errors() as local_errors:
+                    match_args_type = analyze_member_access(
+                        "__match_args__",
+                        typ,
+                        o,
+                        False,
+                        False,
+                        False,
+                        self.msg,
+                        original_type=typ,
+                        chk=self.chk,
+                    )
+                    has_local_errors = local_errors.has_new_errors()
+                if has_local_errors:
+                    self.msg.fail(
+                        message_registry.MISSING_MATCH_ARGS.format(
+                            typ.str_with_options(self.options)
+                        ),
+                        o,
+                    )
+                    return self.early_non_match()
+
+                proper_match_args_type = get_proper_type(match_args_type)
+                if isinstance(proper_match_args_type, TupleType):
+                    match_arg_names = get_match_arg_names(proper_match_args_type)
+
+                    if len(o.positionals) > len(match_arg_names):
+                        self.msg.fail(message_registry.CLASS_PATTERN_TOO_MANY_POSITIONAL_ARGS, o)
+                        return self.early_non_match()
+                else:
+                    match_arg_names = [None] * len(o.positionals)
+
+                for arg_name, pos in zip(match_arg_names, o.positionals):
+                    keyword_pairs.append((arg_name, pos))
+                    if arg_name is not None:
+                        match_arg_set.add(arg_name)
+
+        #
+        # Check for duplicate patterns
+        #
+        keyword_arg_set = set()
+        has_duplicates = False
+        for key, value in zip(o.keyword_keys, o.keyword_values):
+            keyword_pairs.append((key, value))
+            if key in match_arg_set:
+                self.msg.fail(
+                    message_registry.CLASS_PATTERN_KEYWORD_MATCHES_POSITIONAL.format(key), value
+                )
+                has_duplicates = True
+            elif key in keyword_arg_set:
+                self.msg.fail(
+                    message_registry.CLASS_PATTERN_DUPLICATE_KEYWORD_PATTERN.format(key), value
+                )
+                has_duplicates = True
+            keyword_arg_set.add(key)
+
+        if has_duplicates:
+            return self.early_non_match()
+
+        #
+        # Check keyword patterns
+        #
+        can_match = True
+        for keyword, pattern in keyword_pairs:
+            key_type: Type | None = None
+            with self.msg.filter_errors() as local_errors:
+                if keyword is not None:
+                    key_type = analyze_member_access(
+                        keyword,
+                        narrowed_type,
+                        pattern,
+                        False,
+                        False,
+                        False,
+                        self.msg,
+                        original_type=new_type,
+                        chk=self.chk,
+                    )
+                else:
+                    key_type = AnyType(TypeOfAny.from_error)
+                has_local_errors = local_errors.has_new_errors()
+            if has_local_errors or key_type is None:
+                key_type = AnyType(TypeOfAny.from_error)
+                self.msg.fail(
+                    message_registry.CLASS_PATTERN_UNKNOWN_KEYWORD.format(
+                        typ.str_with_options(self.options), keyword
+                    ),
+                    pattern,
+                )
+
+            inner_type, inner_rest_type, inner_captures = self.accept(pattern, key_type)
+            if is_uninhabited(inner_type):
+                can_match = False
+            else:
+                self.update_type_map(captures, inner_captures)
+                if not is_uninhabited(inner_rest_type):
+                    rest_type = current_type
+
+        if not can_match:
+            new_type = UninhabitedType()
+        return PatternType(new_type, rest_type, captures)
+
+    def should_self_match(self, typ: Type) -> bool:
+        typ = get_proper_type(typ)
+        if isinstance(typ, Instance) and typ.type.is_named_tuple:
+            return False
+        for other in self.self_match_types:
+            if is_subtype(typ, other):
+                return True
+        return False
+
+    def can_match_sequence(self, typ: ProperType) -> bool:
+        if isinstance(typ, UnionType):
+            return any(self.can_match_sequence(get_proper_type(item)) for item in typ.items)
+        for other in self.non_sequence_match_types:
+            # We have to ignore promotions, as memoryview should match, but bytes,
+            # which it can be promoted to, shouldn't
+            if is_subtype(typ, other, ignore_promotions=True):
+                return False
+        sequence = self.chk.named_type("typing.Sequence")
+        # If the static type is more general than sequence the actual type could still match
+        return is_subtype(typ, sequence) or is_subtype(sequence, typ)
+
+    def generate_types_from_names(self, type_names: list[str]) -> list[Type]:
+        types: list[Type] = []
+        for name in type_names:
+            try:
+                types.append(self.chk.named_type(name))
+            except KeyError as e:
+                # Some built in types are not defined in all test cases
+                if not name.startswith("builtins."):
+                    raise e
+        return types
+
+    def update_type_map(
+        self, original_type_map: dict[Expression, Type], extra_type_map: dict[Expression, Type]
+    ) -> None:
+        # Calculating this would not be needed if TypeMap directly used literal hashes instead of
+        # expressions, as suggested in the TODO above it's definition
+        already_captured = {literal_hash(expr) for expr in original_type_map}
+        for expr, typ in extra_type_map.items():
+            if literal_hash(expr) in already_captured:
+                node = get_var(expr)
+                self.msg.fail(
+                    message_registry.MULTIPLE_ASSIGNMENTS_IN_PATTERN.format(node.name), expr
+                )
+            else:
+                original_type_map[expr] = typ
+
+    def construct_sequence_child(self, outer_type: Type, inner_type: Type) -> Type:
+        """
+        If outer_type is a child class of typing.Sequence returns a new instance of
+        outer_type, that is a Sequence of inner_type. If outer_type is not a child class of
+        typing.Sequence just returns a Sequence of inner_type
+
+        For example:
+        construct_sequence_child(List[int], str) = List[str]
+
+        TODO: this doesn't make sense. For example if one has class S(Sequence[int], Generic[T])
+        or class T(Sequence[Tuple[T, T]]), there is no way any of those can map to Sequence[str].
+        """
+        proper_type = get_proper_type(outer_type)
+        if isinstance(proper_type, UnionType):
+            types = [
+                self.construct_sequence_child(item, inner_type)
+                for item in proper_type.items
+                if self.can_match_sequence(get_proper_type(item))
+            ]
+            return make_simplified_union(types)
+        sequence = self.chk.named_generic_type("typing.Sequence", [inner_type])
+        if is_subtype(outer_type, self.chk.named_type("typing.Sequence")):
+            proper_type = get_proper_type(outer_type)
+            if isinstance(proper_type, TupleType):
+                proper_type = tuple_fallback(proper_type)
+            assert isinstance(proper_type, Instance)
+            empty_type = fill_typevars(proper_type.type)
+            partial_type = expand_type_by_instance(empty_type, sequence)
+            return expand_type_by_instance(partial_type, proper_type)
+        else:
+            return sequence
+
+    def early_non_match(self) -> PatternType:
+        return PatternType(UninhabitedType(), self.type_context[-1], {})
+
+
+def get_match_arg_names(typ: TupleType) -> list[str | None]:
+    args: list[str | None] = []
+    for item in typ.items:
+        values = try_getting_str_literals_from_type(item)
+        if values is None or len(values) != 1:
+            args.append(None)
+        else:
+            args.append(values[0])
+    return args
+
+
+def get_var(expr: Expression) -> Var:
+    """
+    Warning: this in only true for expressions captured by a match statement.
+    Don't call it from anywhere else
+    """
+    assert isinstance(expr, NameExpr)
+    node = expr.node
+    assert isinstance(node, Var)
+    return node
+
+
+def get_type_range(typ: Type) -> mypy.checker.TypeRange:
+    typ = get_proper_type(typ)
+    if (
+        isinstance(typ, Instance)
+        and typ.last_known_value
+        and isinstance(typ.last_known_value.value, bool)
+    ):
+        typ = typ.last_known_value
+    return mypy.checker.TypeRange(typ, is_upper_bound=False)
+
+
+def is_uninhabited(typ: Type) -> bool:
+    return isinstance(get_proper_type(typ), UninhabitedType)