[ty] Allow values of type None in type expressions

[sharkdp]

Summary

Allow values of type None in type expressions. The typing spec could be more explicit on whether this is actually allowed or not, but it seems relatively harmless and does help in some use cases like:

try:
    from module import MyClass
except ImportError:
    MyClass = None  # ty: ignore


def f(m: MyClass):
    pass

Test Plan

Updated tests, ecosystem check.

[github-actions]

Diagnostic diff on typing conformance tests

No changes detected when running ty on typing conformance tests ✅

[github-actions]

mypy_primer results

Changes were detected when running on open source projects

kopf (https://github.com/nolar/kopf)
- kopf/_cogs/helpers/thirdparty.py:41:36: error[invalid-type-form] Variable of type `None` is not allowed in a type expression
- kopf/_cogs/helpers/thirdparty.py:45:36: error[invalid-type-form] Variable of type `None` is not allowed in a type expression
- Found 47 diagnostics
+ Found 45 diagnostics

mypy (https://github.com/python/mypy)
- mypy/typeshed/stdlib/tty.pyi:25:43: error[invalid-type-form] Variable of type `None` is not allowed in a type expression
- mypy/typeshed/stdlib/tty.pyi:26:46: error[invalid-type-form] Variable of type `None` is not allowed in a type expression
- Found 1839 diagnostics
+ Found 1837 diagnostics

prefect (https://github.com/PrefectHQ/prefect)
+ src/integrations/prefect-dbt/prefect_dbt/cli/credentials.py:166:42: warning[possibly-missing-attribute] Attribute `get_configs` may be missing on object of type `Unknown | None`
- src/integrations/prefect-dbt/prefect_dbt/cli/credentials.py:133:23: error[invalid-type-form] Variable of type `None` is not allowed in a type expression
- src/integrations/prefect-dbt/prefect_dbt/cli/credentials.py:134:23: error[invalid-type-form] Variable of type `None` is not allowed in a type expression
- src/integrations/prefect-dbt/prefect_dbt/cli/credentials.py:135:23: error[invalid-type-form] Variable of type `None` is not allowed in a type expression
- Found 3308 diagnostics
+ Found 3306 diagnostics

aiohttp (https://github.com/aio-libs/aiohttp)
- aiohttp/client_exceptions.py:161:28: error[invalid-type-form] Variable of type `None` is not allowed in a type expression
- Found 161 diagnostics
+ Found 160 diagnostics

dd-trace-py (https://github.com/DataDog/dd-trace-py)
+ ddtrace/internal/ci_visibility/coverage.py:105:12: warning[possibly-missing-attribute] Attribute `_collector` may be missing on object of type `Unknown | None`
+ ddtrace/internal/ci_visibility/coverage.py:105:44: warning[possibly-missing-attribute] Attribute `_collector` may be missing on object of type `Unknown | None`
+ ddtrace/internal/ci_visibility/coverage.py:127:9: warning[possibly-missing-attribute] Attribute `switch_context` may be missing on object of type `Unknown | None`
- ddtrace/internal/ci_visibility/coverage.py:104:45: error[invalid-type-form] Variable of type `None` is not allowed in a type expression
- ddtrace/internal/ci_visibility/coverage.py:113:20: error[invalid-type-form] Variable of type `None` is not allowed in a type expression
- ddtrace/internal/ci_visibility/coverage.py:125:44: warning[unused-ignore-comment] Unused blanket `type: ignore` directive
- ddtrace/internal/ci_visibility/coverage.py:134:20: error[invalid-type-form] Variable of type `None` is not allowed in a type expression
- ddtrace/internal/ci_visibility/coverage.py:163:44: warning[unused-ignore-comment] Unused blanket `type: ignore` directive
- ddtrace/internal/ci_visibility/coverage.py:178:22: error[invalid-type-form] Variable of type `None` is not allowed in a type expression
+ ddtrace/internal/ci_visibility/coverage.py:179:12: warning[possibly-missing-attribute] Attribute `_collector` may be missing on object of type `Unknown | None`
+ ddtrace/internal/ci_visibility/coverage.py:179:39: warning[possibly-missing-attribute] Attribute `_collector` may be missing on object of type `Unknown | None`
- ddtrace/internal/ci_visibility/coverage.py:188:29: error[invalid-type-form] Variable of type `None` is not allowed in a type expression
+ ddtrace/internal/ci_visibility/coverage.py:184:26: warning[possibly-missing-attribute] Attribute `_collector` may be missing on object of type `Unknown | None`
- Found 8156 diagnostics
+ Found 8155 diagnostics

No memory usage changes detected ✅

[github-actions]

ecosystem-analyzer results

Lint rule	Added	Removed	Changed
invalid-type-form	0	13	0
possibly-missing-attribute	7	0	0
unused-ignore-comment	0	2	0
Total	7	15	0

Full report with detailed diff (timing results)