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Argument 1 to type_check_only has incompatible type type[S[T, *Ts]]; expected type[S[T, *Ts]] #19093

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#19100
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Argument 1 to type_check_only has incompatible type type[S[T, *Ts]]; expected type[S[T, *Ts]]#19093
#19100
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bugmypy got something wrongtopic-pep-646PEP 646 (TypeVarTuple, Unpack)
@jorenham

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@jorenham
jorenham
opened on May 16, 2025 · edited by jorenham
Contributor

Consider this .pyi code snippet:

from typing import Any, Generic, Self, TypeVar, TypeVarTuple, type_check_only

T = TypeVar("T")
Ts = TypeVarTuple("Ts") 

@type_check_only
class S(tuple[*Ts], Generic[T, *Ts]):
    def f(self, x: T, /) -> T: ...

When ran through mypy (checked with 1.15.0 and the master branch), the following error is reported

Argument 1 to "type_check_only" has incompatible type "type[S[T, *Ts]]"; expected "type[S[T, *Ts]]"  [arg-type]

mypy playground repro

Without the additional T type parameter, the false positive error disappears. It also disappears if I remove the tuple baseclass.
And as far as I could tell, it doesn't seem to be associated with a particular config option or cli flag.

Activity

jorenham
added
bugmypy got something wrong
on May 16, 2025
jorenham
changed the title [-]Argument 1 to "type_check_only" has incompatible type "type[S[T, *Ts]]"; expected "type[S[T, *Ts]]"[/-] [+]Argument 1 to `type_check_only` has incompatible type `type[S[T, *Ts]]`; expected `type[S[T, *Ts]]`[/+] on May 16, 2025
sterliakov

sterliakov commented on May 17, 2025

@sterliakov
sterliakov
on May 17, 2025
Collaborator

Amazing. mypy does not support type_check_only at all, but this bug is rather surprising - we must be doing something deeply wrong.

Here's a pdb session following quick debugging:

> /home/stas/Documents/Work/mypy/mypy/messages.py(787)incompatible_argument()
-> breakpoint()
(Pdb) arg_type
def [T, Ts] (typing.Iterable[builtins.object] =) -> tuple[Unpack[Ts`2], fallback=a.S[T`1, Unpack[Ts`2]]]
(Pdb) expected_type
def [T, Ts] (typing.Iterable[builtins.object] =) -> tuple[Unpack[Ts`2], fallback=a.S[T`1, Unpack[Ts`2]]]
(Pdb) arg_type==expected_type
True
(Pdb) up
> /home/stas/Documents/Work/mypy/mypy/checkexpr.py(2650)check_arg()
-> code = self.msg.incompatible_argument(
(Pdb) caller_type
def [T, Ts] (typing.Iterable[builtins.object] =) -> tuple[Unpack[Ts`2], fallback=a.S[T`1, Unpack[Ts`2]]]
(Pdb) callee_type
def [T, Ts] (typing.Iterable[builtins.object] =) -> tuple[Unpack[Ts`2], fallback=a.S[T`1, Unpack[Ts`2]]]
(Pdb) caller_type==callee_type
True
(Pdb) is_subtype(caller_type, callee_type, options=self.chk.options)
False
(Pdb) is_subtype(caller_type, callee_type)
False

Hm, so the type is not a subtype of itself. I suspect fixing this is_subtype behaviour may incidentally close a couple more typevartuple issues.

sterliakov
mentioned this on May 17, 2025
jorenham
mentioned this on May 18, 2025
sterliakov
added
topic-pep-646PEP 646 (TypeVarTuple, Unpack)
on May 19, 2025
sterliakov
added a commit that references this issue on May 29, 2025

Merge branch 'master' into bugfix/pythongh-19093-infer-constraints-in…

0fdd0bc
hauntsaninja
added a commit that references this issue on Jun 18, 2025

Include tuple fallback in constraints built from tuple types (#19100)

4cda52d
hauntsaninja
closed this as completedin #19100on Jun 18, 2025
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        Argument 1 to `type_check_only` has incompatible type `type[S[T, *Ts]]`; expected `type[S[T, *Ts]]` · Issue #19093 · python/mypy