Regression for type variables bound to literals

[RobertCraigie]

Describe the bug

This snippet worked prior to 1.1.289

from __future__ import annotations

from typing import Literal, TypeVar, Mapping

_T = TypeVar('_T', bound=Literal['country', 'city'])

def foo(by: list[_T], order: Mapping[_T, str] | None = None) -> None:
    ...

foo(['country'])
foo(['country'], order={'country': 'asc'})

Pyright seems to infer the by and order arguments to list[str] and dict[str, str] which is not correct.

foo.py
 foo.py:13:6 - error: Argument of type "list[str]" cannot be assigned to parameter "by" of type "list[_T@foo]" in function "foo"
    Type "Literal['country']" cannot be assigned to type "_T@foo" (reportGeneralTypeIssues)
  foo.py:14:6 - error: Argument of type "list[str]" cannot be assigned to parameter "by" of type "list[_T@foo]" in function "foo"
    Type "Literal['country']" cannot be assigned to type "_T@foo" (reportGeneralTypeIssues)
  foo.py:14:24 - error: Argument of type "dict[str, str]" cannot be assigned to parameter "order" of type "Mapping[_T@foo, str] | None" in function "foo"
    Type "dict[str, str]" cannot be assigned to type "Mapping[_T@foo, str] | None"
      "dict[str, str]" is incompatible with "Mapping[_T@foo, str]"
        TypeVar "_KT@Mapping" is invariant
          Type "str" cannot be assigned to type "_T@foo"
      Type cannot be assigned to type "None" (reportGeneralTypeIssues)
3 errors, 0 warnings, 0 informations

Expected behavior

The given snippet should pass type checks.

VS Code extension or command-line

Command-line v1.1.289

[erictraut]

This was an intentional change, not a regression. It was needed to fix another issue in the constraint solver.

Mypy also doesn't accept the above code, so I thought it was unlikely that anyone was relying on the need to bind a type variable to a literal type.

Can you explain the scenario where a type variable bound to a literal (or a union of literals) is useful?

[RobertCraigie]

In my case it's used to represent the relationships between the arguments, e.g. you cannot specify a field in order that you haven't included in by.

results = await Profile.prisma().group_by(
    ['country'],
    order={
        # city has to be included in the `by` argument to be valid
        'city': 'asc',
    }
)

[erictraut]

This will be addressed in the next release.

[RobertCraigie]

Amazing, thank you so much!

[erictraut]

This is included in pyright 1.1.290, which I just published. It will also be included in a future release of pylance.