@@ -124,10 +124,10 @@ class TypingChecker(BaseChecker):
                 "type": "yn",
                 "metavar": "<y_or_n>",
                 "help": (
-                    "Set to ``no`` if the app / libary does NOT need to "
-                    "support runtime introspection of type "
-                    "annotations. Only applies to Python version "
-                    "3.7 - 3.9"
+                    "Set to ``no`` if the app / library does NOT need to "
+                    "support runtime introspection of type annotations. "
+                    "Only change it if you understand what that means. "
+                    "Applies to Python version 3.7 - 3.9"
                 ),
             },
         ),
from functools import lru_cache
from typing import Dict, List, NamedTuple, Set, Union

import astroid
import astroid.bases
import astroid.node_classes

from pylint.checkers import BaseChecker
from pylint.checkers.utils import (
    check_messages,
    is_node_in_type_annotation_context,
    safe_infer,
)
from pylint.interfaces import IAstroidChecker
from pylint.lint import PyLinter


class TypingAlias(NamedTuple):
    name: str
    name_collision: bool


DEPRECATED_TYPING_ALIASES: Dict[str, TypingAlias] = {
    "typing.Tuple": TypingAlias("tuple", False),
    "typing.List": TypingAlias("list", False),
    "typing.Dict": TypingAlias("dict", False),
    "typing.Set": TypingAlias("set", False),
    "typing.FrozenSet": TypingAlias("frozenset", False),
    "typing.Type": TypingAlias("type", False),
    "typing.Deque": TypingAlias("collections.deque", True),
    "typing.DefaultDict": TypingAlias("collections.defaultdict", True),
    "typing.OrderedDict": TypingAlias("collections.OrderedDict", True),
    "typing.Counter": TypingAlias("collections.Counter", True),
    "typing.ChainMap": TypingAlias("collections.ChainMap", True),
    "typing.Awaitable": TypingAlias("collections.abc.Awaitable", True),
    "typing.Coroutine": TypingAlias("collections.abc.Coroutine", True),
    "typing.AsyncIterable": TypingAlias("collections.abc.AsyncIterable", True),
    "typing.AsyncIterator": TypingAlias("collections.abc.AsyncIterator", True),
    "typing.AsyncGenerator": TypingAlias("collections.abc.AsyncGenerator", True),
    "typing.Iterable": TypingAlias("collections.abc.Iterable", True),
    "typing.Iterator": TypingAlias("collections.abc.Iterator", True),
    "typing.Generator": TypingAlias("collections.abc.Generator", True),
    "typing.Reversible": TypingAlias("collections.abc.Reversible", True),
    "typing.Container": TypingAlias("collections.abc.Container", True),
    "typing.Collection": TypingAlias("collections.abc.Collection", True),
    "typing.Callable": TypingAlias("collections.abc.Callable", True),
    "typing.AbstractSet": TypingAlias("collections.abc.Set", False),
    "typing.MutableSet": TypingAlias("collections.abc.MutableSet", True),
    "typing.Mapping": TypingAlias("collections.abc.Mapping", True),
    "typing.MutableMapping": TypingAlias("collections.abc.MutableMapping", True),
    "typing.Sequence": TypingAlias("collections.abc.Sequence", True),
    "typing.MutableSequence": TypingAlias("collections.abc.MutableSequence", True),
    "typing.ByteString": TypingAlias("collections.abc.ByteString", True),
    "typing.MappingView": TypingAlias("collections.abc.MappingView", True),
    "typing.KeysView": TypingAlias("collections.abc.KeysView", True),
    "typing.ItemsView": TypingAlias("collections.abc.ItemsView", True),
    "typing.ValuesView": TypingAlias("collections.abc.ValuesView", True),
    "typing.ContextManager": TypingAlias("contextlib.AbstractContextManager", False),
    "typing.AsyncContextManager": TypingAlias(
        "contextlib.AbstractAsyncContextManager", False
    ),
    "typing.Pattern": TypingAlias("re.Pattern", True),
    "typing.Match": TypingAlias("re.Match", True),
    "typing.Hashable": TypingAlias("collections.abc.Hashable", True),
    "typing.Sized": TypingAlias("collections.abc.Sized", True),
}

ALIAS_NAMES = frozenset(key.split(".")[1] for key in DEPRECATED_TYPING_ALIASES)
UNION_NAMES = ("Optional", "Union")


class DeprecatedTypingAliasMsg(NamedTuple):
    node: Union[astroid.Name, astroid.Attribute]
    qname: str
    alias: str
    parent_subscript: bool


class TypingChecker(BaseChecker):
    """Find issue specifically related to type annotations."""

    __implements__ = (IAstroidChecker,)

    name = "typing"
    priority = -1
    msgs = {
        "W6001": (
            "'%s' is deprecated, use '%s' instead",
            "deprecated-typing-alias",
            "Emitted when a deprecated typing alias is used.",
        ),
        "R6002": (
            "'%s' will be deprecated with PY39, consider using '%s' instead%s",
            "consider-using-alias",
            "Only emitted if 'runtime-typing=no' and a deprecated "
            "typing alias is used in a type annotation context in "
            "Python 3.7 or 3.8.",
        ),
        "R6003": (
            "Consider using alternative Union syntax instead of '%s'%s",
            "consider-alternative-union-syntax",
            "Emitted when 'typing.Union' or 'typing.Optional' is used "
            "instead of the alternative Union syntax 'int | None'.",
        ),
    }
    options = (
        (
            "py-version",
            {
                "default": (3, 7),
                "type": "py_version",
                "metavar": "<py_version>",
                "help": (
                    "Min Python version to use for typing related checks, "
                    "e.g. ``3.7``. This should be equal to the min supported Python "
                    "version of the project."
                ),
            },
        ),
        (
            "runtime-typing",
            {
                "default": True,
                "type": "yn",
                "metavar": "<y_or_n>",
                "help": (
                    "Set to ``no`` if the app / libary does NOT need to "
                    "support runtime introspection of type "
                    "annotations. Only applies to Python version "
                    "3.7 - 3.9"
                ),
            },
        ),
    )

    def __init__(self, linter: PyLinter) -> None:
        """Initialize checker instance."""
        super().__init__(linter=linter)
        self._alias_name_collisions: Set[str] = set()
        self._consider_using_alias_msgs: List[DeprecatedTypingAliasMsg] = []

    @lru_cache()
    def _py37_plus(self) -> bool:
        return self.config.py_version >= (3, 7)

    @lru_cache()
    def _py39_plus(self) -> bool:
        return self.config.py_version >= (3, 9)

    @lru_cache()
    def _py310_plus(self) -> bool:
        return self.config.py_version >= (3, 10)

    @lru_cache()
    def _should_check_typing_alias(self) -> bool:
        """The use of type aliases (PEP 585) requires Python 3.9
        or Python 3.7+ with postponed evaluation.
        """
        return (
            self._py39_plus()
            or self._py37_plus()
            and self.config.runtime_typing is False
        )

    @lru_cache()
    def _should_check_alternative_union_syntax(self) -> bool:
        """The use of alternative union syntax (PEP 604) requires Python 3.10
        or Python 3.7+ with postponed evaluation.
        """
        return (
            self._py310_plus()
            or self._py37_plus()
            and self.config.runtime_typing is False
        )

    def _msg_postponed_eval_hint(self, node) -> str:
        """Message hint if postponed evaluation isn't enabled."""
        if self._py310_plus() or "annotations" in node.root().future_imports:
            return ""
        return ". Add 'from __future__ import annotations' as well"

    @check_messages(
        "deprecated-typing-alias",
        "consider-using-alias",
        "consider-alternative-union-syntax",
    )
    def visit_name(self, node: astroid.Name) -> None:
        if self._should_check_typing_alias() and node.name in ALIAS_NAMES:
            self._check_for_typing_alias(node)
        if self._should_check_alternative_union_syntax() and node.name in UNION_NAMES:
            self._check_for_alternative_union_syntax(node, node.name)

    @check_messages(
        "deprecated-typing-alias",
        "consider-using-alias",
        "consider-alternative-union-syntax",
    )
    def visit_attribute(self, node: astroid.Attribute):
        if self._should_check_typing_alias() and node.attrname in ALIAS_NAMES:
            self._check_for_typing_alias(node)
        if (
            self._should_check_alternative_union_syntax()
            and node.attrname in UNION_NAMES
        ):
            self._check_for_alternative_union_syntax(node, node.attrname)

    def _check_for_alternative_union_syntax(
        self,
        node: Union[astroid.Name, astroid.Attribute],
        name: str,
    ) -> None:
        """Check if alternative union syntax could be used.

        Requires
        - Python 3.10
        - OR: Python 3.7+ with postponed evaluation in
              a type annotation context
        """
        inferred = safe_infer(node)
        if not (
            isinstance(inferred, astroid.FunctionDef)
            and inferred.qname()
            in (
                "typing.Optional",
                "typing.Union",
            )
            or isinstance(inferred, astroid.bases.Instance)
            and inferred.qname() == "typing._SpecialForm"
        ):
            return
        if not (self._py310_plus() or is_node_in_type_annotation_context(node)):
            return
        self.add_message(
            "consider-alternative-union-syntax",
            node=node,
            args=(name, self._msg_postponed_eval_hint(node)),
        )

    def _check_for_typing_alias(
        self,
        node: Union[astroid.Name, astroid.Attribute],
    ) -> None:
        """Check if typing alias is depecated or could be replaced.

        Requires
        - Python 3.9
        - OR: Python 3.7+ with postponed evaluation in
              a type annotation context

        For Python 3.7+: Only emitt message if change doesn't create
            any name collisions, only ever used in a type annotation
            context, and can safely be replaced.
        """
        inferred = safe_infer(node)
        if not isinstance(inferred, astroid.ClassDef):
            return
        alias = DEPRECATED_TYPING_ALIASES.get(inferred.qname(), None)
        if alias is None:
            return

        if self._py39_plus():
            self.add_message(
                "deprecated-typing-alias",
                node=node,
                args=(inferred.qname(), alias.name),
            )
            return

        # For PY37+, check for type annotation context first
        if not is_node_in_type_annotation_context(node) and isinstance(
            node.parent, astroid.Subscript
        ):
            if alias.name_collision is True:
                self._alias_name_collisions.add(inferred.qname())
            return
        self._consider_using_alias_msgs.append(
            DeprecatedTypingAliasMsg(
                node,
                inferred.qname(),
                alias.name,
                isinstance(node.parent, astroid.Subscript),
            )
        )

    @check_messages("consider-using-alias")
    def leave_module(self, node: astroid.Module) -> None:
        """After parsing of module is complete, add messages for
        'consider-using-alias' check. Make sure results are safe
        to recommend / collision free.
        """
        if self._py37_plus() and not self._py39_plus():
            msg_future_import = self._msg_postponed_eval_hint(node)
            while True:
                try:
                    msg = self._consider_using_alias_msgs.pop(0)
                except IndexError:
                    break
                if msg.qname in self._alias_name_collisions:
                    continue
                self.add_message(
                    "consider-using-alias",
                    node=msg.node,
                    args=(
                        msg.qname,
                        msg.alias,
                        msg_future_import if msg.parent_subscript else "",
                    ),
                )
        # Clear all module cache variables
        self._alias_name_collisions.clear()
        self._consider_using_alias_msgs.clear()


def register(linter: PyLinter) -> None:
    linter.register_checker(TypingChecker(linter))

Could we explain "what that means" or link to the doc that explains it here ? :)