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[ty] Add support for typing.Concatenate #23689

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[ty] Add support for typing.Concatenate #23689

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399ef75
Add support for `typing.Concatenate`
dhruvmanila Mar 16, 2026
cc94ead
fix some assignability cases
dhruvmanila Mar 16, 2026
2e7414a
simplify display impl for now
dhruvmanila Mar 16, 2026
4d0ef38
update concatenate test cases
dhruvmanila Mar 16, 2026
35b7ce3
update other test cases
dhruvmanila Mar 17, 2026
77f0865
rename `find_paramspec_*` to `as_paramspec_with_prefix`
dhruvmanila Mar 17, 2026
a56d077
fix panic
dhruvmanila Mar 17, 2026
f584a56
Merge branch 'main' into dhruv/typing-concatenate
dhruvmanila Mar 18, 2026
dae25e3
add regression test cases from ecosystem analysis
dhruvmanila Mar 18, 2026
b9181f6
run prek
dhruvmanila Mar 18, 2026
46af3bb
remove stale code
dhruvmanila Mar 18, 2026
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8 changes: 3 additions & 5 deletions crates/ty_python_semantic/resources/mdtest/annotations/callable.md
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Original file line number Diff line number Diff line change
Expand Up @@ -369,16 +369,14 @@ Using `Concatenate` as the first argument to `Callable`:
from typing_extensions import Callable, Concatenate

def _(c: Callable[Concatenate[int, str, ...], int]):
# TODO: Should reveal the correct signature
reveal_type(c) # revealed: (...) -> int
reveal_type(c) # revealed: (int, str, /, *args: Any, **kwargs: Any) -> int
```

Other type expressions can be nested inside `Concatenate`:

```py
def _(c: Callable[[Concatenate[int | str, type[str], ...], int], int]):
# TODO: Should reveal the correct signature
reveal_type(c) # revealed: (...) -> int
def _(c: Callable[Concatenate[int | str, type[str], ...], int]):
reveal_type(c) # revealed: (int | str, type[str], /, *args: Any, **kwargs: Any) -> int
```

But providing fewer than 2 arguments to `Concatenate` is an error:
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2 changes: 1 addition & 1 deletion ...es/ty_python_semantic/resources/mdtest/annotations/unsupported_special_forms.md
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Expand Up @@ -60,7 +60,7 @@ def _(
a: Unpack, # error: [invalid-type-form] "`typing.Unpack` requires exactly one argument when used in a type expression"
b: TypeGuard, # error: [invalid-type-form] "`typing.TypeGuard` requires exactly one argument when used in a type expression"
c: TypeIs, # error: [invalid-type-form] "`typing.TypeIs` requires exactly one argument when used in a type expression"
d: Concatenate, # error: [invalid-type-form] "`typing.Concatenate` requires at least two arguments when used in a type expression"
d: Concatenate, # error: [invalid-type-form] "`typing.Concatenate` is not allowed in this context in a type expression"
e: ParamSpec,
f: Generic, # error: [invalid-type-form] "`typing.Generic` is not allowed in type expressions"
) -> None:
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32 changes: 32 additions & 0 deletions crates/ty_python_semantic/resources/mdtest/call/callables_as_descriptors.md
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Expand Up @@ -203,6 +203,38 @@ class Calculator:
reveal_type(Calculator().square_then_round(3.14)) # revealed: Unknown | int
```

## Use case: Wrappers with explicit receivers
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@dhruvmanila dhruvmanila Mar 18, 2026 •
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This seems similar to astral-sh/ty#491

Reference in trio: https://github.com/python-trio/trio/blob/8ecba055808f09dae9ee4b232e628696ef53eb04/src/trio/_path.py#L72-L79


`trio` defines multiple functions that takes in a callable with `Concatenate`-prepended receiver
types, and returns a wrapper function with a different receiver type. They should still preserve
descriptor behavior when the returned callable is assigned in the class body.

```py
from collections.abc import Callable, Iterable
from typing import Concatenate, ParamSpec, TypeVar

P = ParamSpec("P")
T = TypeVar("T")

class RawPath:
def write_bytes(self, data: bytes) -> int:
raise NotImplementedError

def _wrap_method(
fn: Callable[Concatenate[RawPath, P], T],
) -> Callable[Concatenate["Path", P], T]:
raise NotImplementedError

class Path:
write_bytes = _wrap_method(RawPath.write_bytes)

def check(path: Path) -> None:
# TODO: shouldn't be errors, should reveal `int`
# error: [missing-argument]
# error: [invalid-argument-type]
reveal_type(path.write_bytes(b"")) # revealed: Unknown | int
```

## Use case: Treating dunder methods as bound-method descriptors

pytorch defines a `__pow__` dunder attribute on [`TensorBase`] in a similar way to the following
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2 changes: 1 addition & 1 deletion crates/ty_python_semantic/resources/mdtest/final.md
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Expand Up @@ -1325,7 +1325,7 @@ class Base(ABC):
@abstractproperty # error: [deprecated]
def value(self) -> int:
return 0

# error: [invalid-argument-type]
@abstractclassmethod # error: [deprecated]
def make(cls) -> "Base":
raise NotImplementedError
Comment on lines -1328 to 1331
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@dhruvmanila dhruvmanila Mar 18, 2026 •
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I'm a bit unsure on why this isn't working, it might be related to the fact that it accepts type[_T] as the first parameter. Both mypy and Pyright seems to be raising an error here as well. And, ty works correctly when using the correct way i.e.,

    @classmethod
    @abstractmethod
    def make(cls) -> "Base":
        reveal_type(cls)  # revealed: type[Self@make]
        raise NotImplementedError

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