[ty] Validate PEP 695 type alias scope and redeclaration rules

[charliermarsh]

Summary

We now detect PEP 695 redeclarations in the same scope, like:

type Alias = int
type Alias = str

And also declarations within function scopes:

def f():
    type Local = int

[astral-sh-bot]

Typing conformance results improved 🎉

The percentage of diagnostics emitted that were expected errors increased from 87.72% to 87.75%. The percentage of expected errors that received a diagnostic increased from 82.85% to 83.03%. The number of fully passing files held steady at 74/132.

Summary

How are test cases classified?

Each test case represents one expected error annotation or a group of annotations sharing a tag. Counts are per test case, not per diagnostic — multiple diagnostics on the same line count as one. Required annotations (E) are true positives when ty flags the expected location and false negatives when it does not. Optional annotations (E?) are true positives when flagged but true negatives (not false negatives) when not. Tagged annotations (E[tag]) require ty to flag exactly one of the tagged lines; tagged multi-annotations (E[tag+]) allow any number up to the tag count. Flagging unexpected locations counts as a false positive.

Metric	Old	New	Diff	Outcome
True Positives	879	881	+2	⏫ (✅)
False Positives	123	123	+0
False Negatives	182	180	-2	⏬ (✅)
Total Diagnostics	1052	1054	+2	⏫
Precision	87.72%	87.75%	+0.02%	⏫ (✅)
Recall	82.85%	83.03%	+0.19%	⏫ (✅)
Passing Files	74/132	74/132	+0

Test file breakdown

1 file altered

File	True Positives	False Positives	False Negatives	Status
aliases_type_statement.py	26 (+2) ✅	1	1 (-2) ✅	📈 Improving
Total (all files)	881 (+2) ✅	123	180 (-2) ✅	74/132

True positives added (2)

2 diagnostics

Test case	Diff
aliases_type_statement.py:56	+error[invalid-type-alias] `type` statements are not allowed in function scopes
aliases_type_statement.py:52	+error[invalid-type-alias] Type alias `BadTypeAlias14` is already defined in this scope

[astral-sh-bot]

Memory usage report

Memory usage unchanged ✅

[astral-sh-bot]

ecosystem-analyzer results

Lint rule	Added	Removed	Changed
invalid-type-alias	1	0	0
Total	1	0	0

Raw diff:

AutoSplit (https://github.com/Toufool/AutoSplit)
+ src/utils.py:34:10 error[invalid-type-alias] Type alias `STARTUPINFO` is already defined in this scope

Full report with detailed diff (timing results)

[charliermarsh]

I was a bit surprised by this, but other type checkers do flag it.

[carljm]

I don't like flagging this -- does the conformance suite require it?

[carljm]

If we do flag this, we also need to ensure we don't flag it if one of the branches is statically unreachable.

[AlexWaygood]

I agree with @carljm, but yes -- unfortunately the conformance suite does require it currently: https://github.com/python/typing/blob/1df1565c69730d88ce6877009d268ba1d602af1e/conformance/tests/aliases_type_statement.py#L52. Not sure if that's actually following the spec or if the conformance suite is going beyond the spec there

[carljm]

I can't find any such requirement in PEP 695 or in the spec. I think we should PR the conformance suite to not require this error.

[AlexWaygood]

I also don't see the rule against defining type aliases in function bodies specified anywhere, and that also seems sorta unnecessary... it's not totally clear to me why users should be prohibited from doing something like this, which seems fine: https://play.ty.dev/f918c85d-8bf3-440c-a5bf-1188cd72d053

[carljm]

Just put up python/typing#2249. If that goes through, I think we can close this PR?

[charliermarsh]

Yup!

[dcreager]

To confirm, I'm holding off on reviewing the code on the assumption that we're about to close the PR. If that changes I'll take a look!

[charliermarsh]

That's correct. Given the approvals on that PR, I'm just going to close this out!