Fix union inference of generic class and its generic type

[roberfi]

Fixes #16788

The fix consists in discarding one possible type item of the union if it is an argument of other item that is subtype of the actual type.

Code examples:

• Example 1:

from typing import List, Iterable, TypeVar, Union, reveal_type

T = TypeVar("T")


def coerce_list(x: Union[T, Iterable[T]]) -> List[T]:
    reveal_type(x)
    raise NotImplementedError()


def test() -> None:
    reveal_type(coerce_list(1))
    reveal_type(coerce_list([1, 2]))

Output before the fix:

file.py:7: note: Revealed type is "Union[T`-1, typing.Iterable[T`-1]]"
file.py:12: note: Revealed type is "builtins.list[builtins.int]"
file.py:13: note: Revealed type is "builtins.list[Never]"
file.py:13: error: Argument 1 to "coerce_list" has incompatible type "list[int]"; expected "Iterable[Never]"

Output after the fix:

file.py:7: note: Revealed type is "Union[T`-1, typing.Iterable[T`-1]]"
file.py:12: note: Revealed type is "builtins.list[builtins.int]"
file.py:13: note: Revealed type is "builtins.list[builtins.int]"

• Example 2:

from typing import Generic, TypeVar, Union, reveal_type

T = TypeVar("T")


class GenericClass(Generic[T]):
    def __init__(self, value: T) -> None:
        self.value = value


def method_with_union(arg: Union[GenericClass[T], T]) -> GenericClass[T]:
    if not isinstance(arg, GenericClass):
        arg = GenericClass(arg)
    return arg


result_1 = method_with_union(GenericClass("test"))
reveal_type(result_1)

result_2 = method_with_union("test")
reveal_type(result_2)

Output before the fix:

file.py:17: error: Need type annotation for "result_1"  [var-annotated]
file.py:17: error: Argument 1 to "method_with_union" has incompatible type "GenericClass[str]"; expected "GenericClass[Never]"  [arg-type]
file.py:18: note: Revealed type is "file.GenericClass[Any]"
file.py:21: note: Revealed type is "file.GenericClass[builtins.str]"

Output after the fix:

file.py:18: note: Revealed type is "file.GenericClass[builtins.str]"
file.py:21: note: Revealed type is "file.GenericClass[builtins.str]"

[LevBernstein]

Bumping this for visibility--I'm seeing the same error in my own project, caused by calling a nextcord.Command with a keyword argument:

bb_test.py:2276: error: Argument "words" to "__call__" of "Command" has incompatible type "str"; expected "Never"  [arg-type]
...
ch = MockChannel(guild=MockGuild())
ctx = MockContext(Bot.BeardlessBot, channel=ch)
assert await Bot.cmdDefine(ctx, words="f") == 1
...
@BeardlessBot.command(name="define")  # type: ignore[arg-type]
async def cmdDefine(ctx: misc.BotContext, *, words: str = "") -> int:
if misc.ctxCreatedThread(ctx):
  return -1
emb = await misc.define(words)
await ctx.send(embed=emb)
return 1

The command decorator in that snippet also has a type-ignore comment, because every use of the decorator sees the same "Never" issue. I would love for this to be merged.

[LevBernstein]

Overlapping is spelled with two "p"s; same issue in all occurrences.

[roberfi]

Oh, yes. You are right. Thank you for the advice! I have fixed it

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

Tanjun (https://github.com/FasterSpeeding/Tanjun)
- tanjun/dependencies/data.py:221: error: Argument "callback" to "inject" has incompatible type "Callable[..., Coroutine[Any, Any, _T]]"; expected "Callable[..., Union[Coroutine[Any, Any, Never], Never]]"  [arg-type]
- tanjun/dependencies/data.py:351: error: Argument "callback" to "inject" has incompatible type "Callable[..., Coroutine[Any, Any, _T]]"; expected "Callable[..., Union[Coroutine[Any, Any, Never], Never]]"  [arg-type]
- tanjun/schedules.py:338: error: Argument 1 to "call_with_async_di" of "Client" has incompatible type "_CallbackSigT"; expected "Callable[..., Union[Coroutine[Any, Any, Never], Never]]"  [arg-type]
- tanjun/schedules.py:364: error: Argument 1 to "call_with_async_di" of "Client" has incompatible type "Callable[..., Coroutine[Any, Any, None]]"; expected "Callable[..., Union[Coroutine[Any, Any, Never], Never]]"  [arg-type]
- tanjun/schedules.py:387: error: Argument 1 to "call_with_async_di" of "Client" has incompatible type "Callable[..., Coroutine[Any, Any, None]]"; expected "Callable[..., Union[Coroutine[Any, Any, Never], Never]]"  [arg-type]
- tanjun/schedules.py:1050: error: Argument 1 to "call_with_async_di" of "Client" has incompatible type "_CallbackSigT"; expected "Callable[..., Union[Coroutine[Any, Any, Never], Never]]"  [arg-type]
- tanjun/commands/slash.py:3171: error: Argument 1 to "call_with_async_di" of "Context" has incompatible type "_SlashCallbackSigT"; expected "Callable[..., Union[Coroutine[Any, Any, Never], Never]]"  [arg-type]
- tanjun/commands/slash.py:3213: error: Argument 1 to "call_with_async_di" of "Context" has incompatible type "Callable[[AutocompleteContext, str, VarArg(Any), KwArg(Any)], Coroutine[Any, Any, None]]"; expected "Callable[..., Union[Coroutine[Any, Any, Never], Never]]"  [arg-type]
- tanjun/commands/message.py:346: error: Argument 1 to "call_with_async_di" of "Context" has incompatible type "_MessageCallbackSigT"; expected "Callable[..., Union[Coroutine[Any, Any, Never], Never]]"  [arg-type]
- tanjun/commands/menu.py:685: error: Argument 1 to "call_with_async_di" of "Context" has incompatible type "_AnyMenuCallbackSigT"; expected "Callable[..., Union[Coroutine[Any, Any, Never], Never]]"  [arg-type]
- tanjun/clients.py:2291: error: "Never" has no attribute "__iter__" (not iterable)  [attr-defined]
- tanjun/clients.py:2291: error: Argument 1 to "call_with_async_di" of "Context" has incompatible type "Callable[[MessageContext, VarArg(Any), KwArg(Any)], Coroutine[Any, Any, Iterable[str]]]"; expected "Callable[..., Union[Coroutine[Any, Any, Never], Never]]"  [arg-type]

ibis (https://github.com/ibis-project/ibis)
+ ibis/common/egraph.py:731: error: Item "function" of "Callable[..., Any] | Pattern | Variable" has no attribute "substitute"  [union-attr]
- ibis/common/egraph.py:729: error: Need type annotation for "rewrite"  [var-annotated]
- ibis/common/egraph.py:729: error: Argument 1 to "promote_list" has incompatible type "list[Rewrite]"; expected "Sequence[Never]"  [arg-type]
- ibis/expr/types/relations.py:678: error: Need type annotation for "arg"  [var-annotated]
- ibis/expr/types/relations.py:680: error: Argument 1 to "promote_list" has incompatible type "Sequence[str | int]"; expected "Sequence[Never]"  [arg-type]

discord.py (https://github.com/Rapptz/discord.py)
- discord/app_commands/commands.py:1003: error: Argument 1 to "maybe_coroutine" has incompatible type "Callable[[Interaction[Client]], Coroutine[Any, Any, bool]]"; expected "Callable[[Interaction[Client]], Never | Awaitable[Never]]"  [arg-type]
- discord/ext/commands/cog.py:706: error: Argument 1 to "maybe_coroutine" has incompatible type "Callable[[], Coroutine[Any, Any, None]]"; expected "Callable[[], Never | Awaitable[Never]]"  [arg-type]
- discord/ext/commands/cog.py:723: error: Argument 1 to "maybe_coroutine" has incompatible type "Callable[[], Coroutine[Any, Any, None]]"; expected "Callable[[], Never | Awaitable[Never]]"  [arg-type]
- discord/ext/commands/cog.py:776: error: Argument 1 to "maybe_coroutine" has incompatible type "Callable[[], Coroutine[Any, Any, None]]"; expected "Callable[[], Never | Awaitable[Never]]"  [arg-type]
- discord/ext/commands/hybrid.py:401: error: Argument 1 to "maybe_coroutine" has incompatible type "Callable[[Interaction[Client]], Coroutine[Any, Any, bool]]"; expected "Callable[[Interaction[Client]], Never | Awaitable[Never]]"  [arg-type]