## Summary
Many symbols in typeshed are defined without being declared. For
example:
```pyi
# builtins:
IOError = OSError
# types
LambdaType = FunctionType
NotImplementedType = _NotImplementedType
# typing
Text = str
# random
uniform = _inst.uniform
# optparse
make_option = Option
# all over the place:
_T = TypeVar("_T")
```
Here, we introduce a change that skips widening the public type of these
symbols (by unioning with `Unknown`).
fixes#17032
## Ecosystem analysis
This is difficult to analyze in detail, but I went over most changes and
it looks very favorable to me overall. The diff on the overall numbers
is:
```
errors: 1287 -> 859 (reduction by 428)
warnings: 45 -> 59 (increase by 14)
```
### Removed false positives
`invalid-base` examples:
```diff
- error[lint:invalid-base] /tmp/mypy_primer/projects/pip/src/pip/_vendor/rich/console.py:548:27: Invalid class base with type `Unknown | Literal[_local]` (all bases must be a class, `Any`, `Unknown` or `Todo`)
- error[lint:invalid-base] /tmp/mypy_primer/projects/tornado/tornado/iostream.py:84:25: Invalid class base with type `Unknown | Literal[OSError]` (all bases must be a class, `Any`, `Unknown` or `Todo`)
- error[lint:invalid-base] /tmp/mypy_primer/projects/mitmproxy/test/conftest.py:35:40: Invalid class base with type `Unknown | Literal[_UnixDefaultEventLoopPolicy]` (all bases must be a class, `Any`, `Unknown` or `Todo`)
```
`invalid-exception-caught` examples:
```diff
- error[lint:invalid-exception-caught] /tmp/mypy_primer/projects/cloud-init/cloudinit/cmd/status.py:334:16: Cannot catch object of type `Literal[ProcessExecutionError]` in an exception handler (must be a `BaseException` subclass or a tuple of `BaseException` subclasses)
- error[lint:invalid-exception-caught] /tmp/mypy_primer/projects/jinja/src/jinja2/loaders.py:537:16: Cannot catch object of type `Literal[TemplateNotFound]` in an exception handler (must be a `BaseException` subclass or a tuple of `BaseException` subclasses)
```
`unresolved-reference` examples
7a0265d36e/cloudinit/handlers/jinja_template.py (L120-L123)
(we now understand the `isinstance` narrowing)
```diff
- error[lint:unresolved-attribute] /tmp/mypy_primer/projects/cloud-init/cloudinit/handlers/jinja_template.py:123:16: Type `Exception` has no attribute `errno`
```
`unknown-argument` examples
https://github.com/hauntsaninja/boostedblob/blob/master/boostedblob/request.py#L53
```diff
- error[lint:unknown-argument] /tmp/mypy_primer/projects/boostedblob/boostedblob/request.py:53:17: Argument `connect` does not match any known parameter of bound method `__init__`
```
`unknown-argument`
There are a lot of `__init__`-related changes because we now understand
[`@attr.s`](3d42a6978a/src/attr/__init__.pyi (L387))
as a `@dataclass_transform` annotated symbol. For example:
```diff
- error[lint:unknown-argument] /tmp/mypy_primer/projects/attrs/tests/test_hooks.py:72:18: Argument `x` does not match any known parameter of bound method `__init__`
```
### New false positives
This can happen if a symbol that previously was inferred as `X |
Unknown` was assigned-to, but we don't yet understand the assignability
to `X`:
https://github.com/strawberry-graphql/strawberry/blob/main/strawberry/exceptions/handler.py#L90
```diff
+ error[lint:invalid-assignment] /tmp/mypy_primer/projects/strawberry/strawberry/exceptions/handler.py:90:9: Object of type `def strawberry_threading_exception_handler(args: tuple[type[BaseException], BaseException | None, TracebackType | None, Thread | None]) -> None` is not assignable to attribute `excepthook` of type `(_ExceptHookArgs, /) -> Any`
```
### New true positives
6bbb5519fe/tests/tracer/test_span.py (L714)
```diff
+ error[lint:invalid-argument-type] /tmp/mypy_primer/projects/dd-trace-py/tests/tracer/test_span.py:714:33: Argument to this function is incorrect: Expected `str`, found `Literal[b"\xf0\x9f\xa4\x94"]`
```
### Changed diagnostics
A lot of changed diagnostics because we now show `@Todo(Support for
`typing.TypeVar` instances in type expressions)` instead of `Unknown`
for all kinds of symbols that used a `_T = TypeVar("_T")` as a type. One
prominent example is the `list.__getitem__` method:
`builtins.pyi`:
```pyi
_T = TypeVar("_T") # previously `TypeVar | Unknown`, now just `TypeVar`
# …
class list(MutableSequence[_T]):
# …
@overload
def __getitem__(self, i: SupportsIndex, /) -> _T: ...
# …
```
which causes this change in diagnostics:
```py
xs = [1, 2]
reveal_type(xs[0]) # previously `Unknown`, now `@Todo(Support for `typing.TypeVar` instances in type expressions)`
```
## Test Plan
Updated Markdown tests
e91e2f49db