[ty] avoid fixpoint unioning of types containing current-cycle Divergent

[carljm]

Partially addresses astral-sh/ty#1732

Summary

Don't union the previous type in fixpoint iteration if the previous type contains a Divergent from the current cycle and the latest type does not. The theory here, as outlined by @mtshiba at astral-sh/ty#1732 (comment), is that oscillation can't occur by removing and then reintroducing a Divergent type repeatedly, since Divergent types are only introduced at the start of fixpoint iteration.

Test Plan

Removes a Divergent type from the added mdtest, doesn't otherwise regress any tests.

[astral-sh-bot]

Diagnostic diff on typing conformance tests

No changes detected when running ty on typing conformance tests ✅

[astral-sh-bot]

mypy_primer results

Changes were detected when running on open source projects

spack (https://github.com/spack/spack)
- lib/spack/spack/mirrors/mirror.py:114:16: error[invalid-return-type] Return type does not match returned value: expected `bool`, found `Literal[True] | Unknown | str | Divergent`
+ lib/spack/spack/mirrors/mirror.py:114:16: error[invalid-return-type] Return type does not match returned value: expected `bool`, found `Literal[True] | Unknown | str`
- lib/spack/spack/mirrors/mirror.py:120:16: error[invalid-return-type] Return type does not match returned value: expected `bool`, found `Unknown | str | Divergent`
+ lib/spack/spack/mirrors/mirror.py:120:16: error[invalid-return-type] Return type does not match returned value: expected `bool`, found `Unknown | str`
- lib/spack/spack/mirrors/mirror.py:263:45: error[invalid-argument-type] Argument to bound method `_update_connection_dict` is incorrect: Expected `dict[Unknown, Unknown]`, found `@Todo | str | Divergent`
+ lib/spack/spack/mirrors/mirror.py:263:45: error[invalid-argument-type] Argument to bound method `_update_connection_dict` is incorrect: Expected `dict[Unknown, Unknown]`, found `@Todo | str`

setuptools (https://github.com/pypa/setuptools)
- setuptools/_distutils/command/build_ext.py:270:23: warning[possibly-missing-attribute] Attribute `split` may be missing on object of type `(Unknown & ~AlwaysFalsy) | (list[tuple[Unknown, str] | Unknown] & ~AlwaysFalsy) | (list[Divergent] & ~AlwaysFalsy)`
+ setuptools/_distutils/command/build_ext.py:270:23: warning[possibly-missing-attribute] Attribute `split` may be missing on object of type `(Unknown & ~AlwaysFalsy) | (list[tuple[Unknown, str] | Unknown] & ~AlwaysFalsy)`
- setuptools/_distutils/command/sdist.py:372:58: error[invalid-argument-type] Argument to function `__new__` is incorrect: Expected `str | Buffer | SupportsInt | SupportsIndex | SupportsTrunc`, found `Unknown | Literal[0] | None | list[Unknown | int | None] | list[Divergent]`
+ setuptools/_distutils/command/sdist.py:372:58: error[invalid-argument-type] Argument to function `__new__` is incorrect: Expected `str | Buffer | SupportsInt | SupportsIndex | SupportsTrunc`, found `Unknown | Literal[0] | None | list[Unknown | int | None]`
- setuptools/_distutils/text_file.py:232:44: error[unsupported-operator] Operator `+` is not supported between objects of type `@Todo | int | None | Divergent` and `Literal[1]`
+ setuptools/_distutils/text_file.py:232:44: error[unsupported-operator] Operator `+` is not supported between objects of type `@Todo | int | None` and `Literal[1]`
- setuptools/_distutils/text_file.py:242:41: error[unsupported-operator] Operator `+` is not supported between objects of type `@Todo | int | None | Divergent` and `Literal[1]`
+ setuptools/_distutils/text_file.py:242:41: error[unsupported-operator] Operator `+` is not supported between objects of type `@Todo | int | None` and `Literal[1]`
- setuptools/_vendor/autocommand/autoparse.py:306:5: error[invalid-assignment] Object of type `Unknown & ~None` is not assignable to attribute `func` on type `_Wrapped[(...), Unknown, (argv=None), Unknown] | _Wrapped[(...), Unknown, Divergent, Unknown]`
- setuptools/_vendor/autocommand/autoparse.py:307:5: error[invalid-assignment] Object of type `Unknown` is not assignable to attribute `parser` on type `_Wrapped[(...), Unknown, (argv=None), Unknown] | _Wrapped[(...), Unknown, Divergent, Unknown]`
+ setuptools/_vendor/autocommand/autoparse.py:306:5: error[unresolved-attribute] Unresolved attribute `func` on type `_Wrapped[(...), Unknown, (argv=None), Unknown]`.
+ setuptools/_vendor/autocommand/autoparse.py:307:5: error[unresolved-attribute] Unresolved attribute `parser` on type `_Wrapped[(...), Unknown, (argv=None), Unknown]`.

prefect (https://github.com/PrefectHQ/prefect)
- src/integrations/prefect-aws/tests/test_s3.py:442:21: error[no-matching-overload] No overload matches arguments
- src/integrations/prefect-aws/tests/test_s3.py:449:21: error[no-matching-overload] No overload matches arguments
- src/integrations/prefect-docker/tests/test-project/flow.py:31:9: error[no-matching-overload] No overload matches arguments
- Found 5533 diagnostics
+ Found 5530 diagnostics

scikit-build-core (https://github.com/scikit-build/scikit-build-core)
- src/scikit_build_core/build/wheel.py:98:20: error[no-matching-overload] No overload of bound method `__init__` matches arguments
- Found 42 diagnostics
+ Found 41 diagnostics

pandas-stubs (https://github.com/pandas-dev/pandas-stubs)
+ pandas-stubs/_typing.pyi:1217:16: warning[unused-ignore-comment] Unused blanket `type: ignore` directive
- Found 5118 diagnostics
+ Found 5119 diagnostics

scipy (https://github.com/scipy/scipy)
- scipy/stats/_axis_nan_policy.py:713:9: error[invalid-assignment] Object of type `Signature` is not assignable to attribute `__signature__` on type `_Wrapped[(...), Unknown, (*args, *, _no_deco=Literal[False], **kwds), Unknown] | _Wrapped[(...), Unknown, Divergent, Unknown]`
+ scipy/stats/_axis_nan_policy.py:713:9: error[unresolved-attribute] Unresolved attribute `__signature__` on type `_Wrapped[(...), Unknown, (*args, *, _no_deco=Literal[False], **kwds), Unknown]`.

Memory usage changes were detected when running on open source projects

trio (https://github.com/python-trio/trio)
-     struct metadata = ~11MB
+     struct metadata = ~10MB

[astral-sh-bot]

ecosystem-analyzer results

Lint rule	Added	Removed	Changed
invalid-assignment	0	3	0
unresolved-attribute	3	0	0
invalid-argument-type	0	0	2
invalid-return-type	0	0	2
unsupported-operator	0	0	2
possibly-missing-attribute	0	0	1
unused-ignore-comment	1	0	0
Total	4	3	7

Full report with detailed diff (timing results)

[codspeed-hq]

CodSpeed Performance Report

Merging #21910 will not alter performance

_{Comparing cjm/nodiv (060a617) with main (3f63ea4)}

Summary

✅ 22 untouched
⏩ 30 skipped¹

Footnotes

1. 30 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[carljm]

With #21906, this no longer has any impact on our mdtests. Pushing a rebase to see if it has any ecosystem impact worth pursuing.

[carljm]

Ok, this doesn't seem to hurt performance and there are cases in the ecosystem where it helps, so I think it's worth going ahead with. I'll add at least one example from the ecosystem as an mdtest and then put it up for review.

[carljm]

If I remove the !has_divergent_type_in_cycle(self) requirement, we do get an oscillation in mdtest/regression/1377_iteration_count_mismatch.md. So it is possible to oscillate between types containing a Divergent from the current cycle. But it should not be possible to reintroduce a Divergent from the current cycle once it has been eliminated.

[carljm]

@mtshiba please take a look and let me know what you think.

[mtshiba]

Looks good!