"Parameters cannot be constrained" with generic ParamSpec

[cdce8p]

Crash Report

Ran into this one while implementing generic ParamSpec after #11847 was merged this morning.
/CC: @A5rocks

Traceback

version: 0.950+dev.e7e1db6a5ad3d07a7a4d46cf280e972f2342a90f
Traceback (most recent call last):
  ...
  File "/.../mypy/mypy/subtypes.py", line 927, in is_callable_compatible
    unified = unify_generic_callable(left, right, ignore_return=ignore_return)
  File "/.../mypy/mypy/subtypes.py", line 1176, in unify_generic_callable
    c = mypy.constraints.infer_constraints(
  File "/.../mypy/mypy/constraints.py", line 108, in infer_constraints
    return _infer_constraints(template, actual, direction)
  File "/.../mypy/mypy/constraints.py", line 181, in _infer_constraints
    return template.accept(ConstraintBuilderVisitor(actual, direction))
  File "/.../mypy/mypy/types.py", line 1111, in accept
    return visitor.visit_instance(self)
  File "/.../mypy/mypy/constraints.py", line 551, in visit_instance
    return self.infer_against_any(template.args, actual)
  File "/.../mypy/mypy/constraints.py", line 731, in infer_against_any
    res.extend(infer_constraints(t, any_type, self.direction))
  File "/.../mypy/mypy/constraints.py", line 108, in infer_constraints
    return _infer_constraints(template, actual, direction)
  File "/.../mypy/mypy/constraints.py", line 181, in _infer_constraints
    return template.accept(ConstraintBuilderVisitor(actual, direction))
  File "/.../mypy/mypy/types.py", line 1347, in accept
    return visitor.visit_parameters(self)
  File "/.../mypy/mypy/constraints.py", line 410, in visit_parameters
    raise RuntimeError("Parameters cannot be constrained to")
RuntimeError: Parameters cannot be constrained to

To Reproduce

from __future__ import annotations

from typing import Any, Callable, Generic, TypeVar
from typing_extensions import ParamSpec

_R = TypeVar("_R")
_P = ParamSpec("_P")
_CallableT = TypeVar("_CallableT", bound=Callable[..., Any])


def callback(func: _CallableT) -> _CallableT:
    # do something
    return func

class Job(Generic[_P, _R]):
    def __init__(self, target: Callable[_P, _R]) -> None:
        self.target = target


@callback
def run_job(job: Job[..., _R], *args: Any) -> _R:
    return job.target(*args)

AFAICT the issues is caused by is_callable_compatible when two TypeVar / ParamSpec variables are used for a Generic class. In combination with the bound TypeVar _CallableT.

mypy/mypy/subtypes.py

Lines 925 to 927 in ab1b488

	if left.variables:
	# Apply generic type variables away in left via type inference.
	unified = unify_generic_callable(left, right, ignore_return=ignore_return)

mypy/mypy/constraints.py

Lines 409 to 410 in 75e907d

	def visit_parameters(self, template: Parameters) -> List[Constraint]:
	raise RuntimeError("Parameters cannot be constrained to")

Modifying run_job to either one of these two options resolves the error.

# Don't use TypeVar '_R' inside 'Job' -> only useful for debugging
@callback
def run_job(job: Job[..., None], *args: Any) -> None:
    return job.target(*args)

# A full workaround -> use '_P' directly, even if not used anywhere else.
# It seems to be allowed and acts as '...' / 'Any' in this case anyway.
@callback
def run_job(job: Job[_P, _R], *args: Any) -> _R:
    return job.target(*args)

Your Environment

• Mypy version used: 0.950+dev.e7e1db6a5ad3d07a7a4d46cf280e972f2342a90f - 2022-04-07
• Python version used: 3.10.4