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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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jorenham opened this issue May 16, 2025 · 1 comment · May be fixed by #19100
Open

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

jorenham opened this issue May 16, 2025 · 1 comment · May be fixed by #19100
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bug mypy got something wrong topic-pep-646 PEP 646 (TypeVarTuple, Unpack)

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@jorenham
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jorenham commented May 16, 2025 •
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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.
@jorenham jorenham added the bug mypy got something wrong label May 16, 2025
@jorenham 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]] May 16, 2025
@sterliakov
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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 sterliakov linked a pull request May 17, 2025 that will close this issue
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@jorenham jorenham mentioned this issue May 18, 2025
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@sterliakov sterliakov added the topic-pep-646 PEP 646 (TypeVarTuple, Unpack) label May 19, 2025
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Include tuple fallback in constraints built from tuple types sterliakov/mypy
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