Narrow TypeVar to bounded TypeVar

[ikonst]

Fixes #15151.

When isinstance narrows a TypeVar, it should narrow it to a bounded TypeVar rather than to an Instance, so it could keep functioning as that TypeVar.

[hauntsaninja]

I think I've broken mypy primer, will fix

[tmke8]

This is a great improvement!

There are still a few things that don't work, like this one:

# flags: --warn-unreachable
from typing import Optional, TypeVar

T = TypeVar("T", bound=Optional[int])  # `Union[None, int]` gives the same result

def foo(v: T) -> T:
    if v is None:
        return v  # E: Statement is unreachable  [unreachable]
    return v

but that's maybe a different problem.

[ikonst]

Might be nice to somehow indicate the updated upper bound in the repr

[ikonst]

I've added testIsinstanceAndNarrowTypeVariableIntersection (see 61b71e9) that's currently expected to fail: perhaps same root cause as the failure to narrow optional.

[ikonst]

Surprisingly all mypy_primer results are expected :)

I'm making this do slightly more than originally planned by changing how narrowed TypeVars are stringified.

[ikonst]

@JelleZijlstra This implements the first option I've outlined in #15151 (comment), can take a look?

[JelleZijlstra]

I think I'll pass on this one, sorry. I'm mostly focusing my open-source time on projects other than mypy.

[ikonst]

@ilevkivskyi What do you think?

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

mitmproxy (https://github.com/mitmproxy/mitmproxy)
+ mitmproxy/coretypes/serializable.py:186: error: Unused "type: ignore" comment  [unused-ignore]

steam.py (https://github.com/Gobot1234/steam.py)
- steam/ext/commands/commands.py:287: error: Argument 1 to "append" of "list" has incompatible type "Command[Any, Any, Any]"; expected "Self"  [arg-type]
- steam/ext/commands/commands.py:940: error: Incompatible return value type (got "Command[Any, Any, Any] | MaybeCommandT", expected "MaybeBool | MaybeCommandT")  [return-value]

mypy (https://github.com/python/mypy)
+ mypy/typeops.py:342: error: Incompatible types in assignment (expression has type "CallableType", variable has type "F")  [assignment]

pandas-stubs (https://github.com/pandas-dev/pandas-stubs)
+ tests/__init__.py:30: error: Unused "type: ignore" comment  [unused-ignore]
+ tests/__init__.py:44: error: Unused "type: ignore" comment  [unused-ignore]

[ilevkivskyi]

I propose to close this in favor of #19183 that offers a different approach.