generic bound `TypeVar` broke with `Callable`

```py
from typing import Callable, TypeVar

R = TypeVar("R")
StrFn = TypeVar("StrFn", bound=Callable[[str], R])

def deco(fn: StrFn) -> StrFn:
return fn

@deco
def foo(value: str): # error, incorrect
...

@deco
def bar(value: str, value2: str = ""): # error, incorrect
...

@deco
def baz(value: str, value2: str): # error, correct
...
```

Use the provided Python snippet as the reproducer and run it through basedmypy to compare the diagnostics for foo, bar, and baz. Done means the incorrect errors on foo and bar are resolved while baz retains its expected error.

Written by the indexing model from the issue text.