[ty] defer inference of legacy TypeVar bound/constraints/defaults #20598

carljm · 2025-09-27T01:53:57Z

Summary

This allows us to handle self-referential bounds/constraints/defaults without panicking.

Handles more cases from astral-sh/ty#256

This also changes the way we infer the types of legacy TypeVars. Rather than understanding a constructor call to typing[_extension].TypeVar inside of any (arbitrarily nested) expression, and having to use a special assigned_to field of the semantic index to try to best-effort figure out what name the typevar was assigned to, we instead understand the creation of a legacy TypeVar only in the supported syntactic position (RHS of a simple un-annotated assignment with one target). In any other position, we just infer it as creating an opaque instance of typing.TypeVar. (This behavior matches all other type checkers.)

So we now special-case TypeVar creation in TypeInferenceBuilder, as a special case of an assignment definition, rather than deeper inside call binding. This does mean we re-implement slightly more of argument-parsing, but in practice this is minimal and easy to handle correctly.

This is easier to implement if we also make the RHS of a simple (no unpacking) one-target assignment statement no longer a standalone expression. Which is fine to do, because simple one-target assignments don't need to infer the RHS more than once. This is a bonus performance (0-3% across various projects) and significant memory-usage win, since most assignment statements are simple one-target assignment statements, meaning we now create many fewer standalone-expression salsa ingredients.

This change does mean that inference of manually-constructed TypeAliasType instances can no longer find its Definition in assigned_to, which regresses go-to-definition for these aliases. In a future PR, TypeAliasType will receive the same treatment that TypeVar did in this PR (moving its special-case inference into TypeInferenceBuilder and supporting it only in the correct syntactic position, and lazily inferring its value type to support recursion), which will also fix the go-to-definition regression. (I decided a temporary edge-case regression is better in this case than doubling the size of this PR.)

This PR also tightens up and fixes various aspects of the validation of TypeVar creation, as seen in the tests.

We still (for now) treat all typevars as instances of typing.TypeVar, even if they were created using typing_extensions.TypeVar. This means we'll wrongly error on e.g. T.__default__ on Python 3.11, even if T is a typing_extensions.TypeVar instance at runtime. We share this wrong behavior with both mypy and pyrefly. It will be easier to fix after we pull in python/typeshed#14840.

There are some issues that showed up here with typevar identity and MarkTypeVarsInferable; the fix here (using the new original field and is_identical_to methods on BoundTypeVarInstance and TypeVarInstance) is a bit kludgy, but it can go away when we eliminate MarkTypeVarsInferable.

Test Plan

Added and updated mdtests.

Conformance suite impact

The impact here is all positive:

We now correctly error on a legacy TypeVar with exactly one constraint type given.
We now correctly error on a legacy TypeVar with both an upper bound and constraints specified.

Ecosystem impact

Basically none; in the setuptools case we just issue slightly different errors on an invalid TypeVar definition, due to the modified validation code.

github-actions · 2025-09-27T01:55:45Z

Diagnostic diff on typing conformance tests

Changes were detected when running ty on typing conformance tests

--- old-output.txt	2025-10-09 21:06:16.051150791 +0000
+++ new-output.txt	2025-10-09 21:06:19.328159431 +0000
@@ -1,6 +1,6 @@
 WARN ty is pre-release software and not ready for production use. Expect to encounter bugs, missing features, and fatal errors.
 fatal[panic] Panicked at /home/runner/.cargo/git/checkouts/salsa-e6f3bb7c2a062968/29ab321/src/function/execute.rs:217:25 when checking `/home/runner/work/ruff/ruff/typing/conformance/tests/aliases_type_statement.py`: `PEP695TypeAliasType < 'db >::value_type_(Id(cc17)): execute: too many cycle iterations`
-fatal[panic] Panicked at /home/runner/.cargo/git/checkouts/salsa-e6f3bb7c2a062968/29ab321/src/function/execute.rs:217:25 when checking `/home/runner/work/ruff/ruff/typing/conformance/tests/aliases_typealiastype.py`: `infer_definition_types(Id(1643f)): execute: too many cycle iterations`
+fatal[panic] Panicked at /home/runner/.cargo/git/checkouts/salsa-e6f3bb7c2a062968/29ab321/src/function/execute.rs:217:25 when checking `/home/runner/work/ruff/ruff/typing/conformance/tests/aliases_typealiastype.py`: `infer_definition_types(Id(1603f)): execute: too many cycle iterations`
 _directives_deprecated_library.py:15:31: error[invalid-return-type] Function always implicitly returns `None`, which is not assignable to return type `int`
 _directives_deprecated_library.py:30:26: error[invalid-return-type] Function always implicitly returns `None`, which is not assignable to return type `str`
 _directives_deprecated_library.py:36:41: error[invalid-return-type] Function always implicitly returns `None`, which is not assignable to return type `Self@__add__`
@@ -364,6 +364,7 @@
 generics_base_class.py:49:22: error[too-many-positional-arguments] Too many positional arguments to class `LinkedList`: expected 1, got 2
 generics_base_class.py:61:18: error[too-many-positional-arguments] Too many positional arguments to class `MyDict`: expected 1, got 2
 generics_basic.py:34:12: error[unsupported-operator] Operator `+` is unsupported between objects of type `AnyStr@concat` and `AnyStr@concat`
+generics_basic.py:49:44: error[invalid-legacy-type-variable] A `TypeVar` cannot have exactly one constraint
 generics_basic.py:139:5: error[type-assertion-failure] Argument does not have asserted type `int`
 generics_basic.py:140:5: error[type-assertion-failure] Argument does not have asserted type `int`
 generics_basic.py:157:5: error[invalid-argument-type] Method `__getitem__` of type `bound method MyMap1[str, int].__getitem__(key: str, /) -> int` cannot be called with key of type `Literal[0]` on object of type `MyMap1[str, int]`
@@ -575,7 +576,8 @@
 generics_upper_bound.py:43:1: error[type-assertion-failure] Argument does not have asserted type `list[int] | set[int]`
 generics_upper_bound.py:51:8: error[invalid-argument-type] Argument to function `longer` is incorrect: Argument type `Literal[3]` does not satisfy upper bound `Sized` of type variable `ST`
 generics_upper_bound.py:51:11: error[invalid-argument-type] Argument to function `longer` is incorrect: Argument type `Literal[3]` does not satisfy upper bound `Sized` of type variable `ST`
-generics_variance.py:14:6: error[invalid-legacy-type-variable] A legacy `typing.TypeVar` cannot be both covariant and contravariant
+generics_upper_bound.py:56:10: error[invalid-legacy-type-variable] A `TypeVar` cannot have both a bound and constraints
+generics_variance.py:14:6: error[invalid-legacy-type-variable] A `TypeVar` cannot be both covariant and contravariant
 generics_variance.py:26:27: error[invalid-return-type] Function always implicitly returns `None`, which is not assignable to return type `Iterator[T_co@ImmutableList]`
 generics_variance.py:57:28: error[invalid-return-type] Function always implicitly returns `None`, which is not assignable to return type `B_co@func`
 generics_variance.py:175:25: error[non-subscriptable] Cannot subscript object of type `<class 'Contra[typing.TypeVar]'>` with no `__class_getitem__` method
@@ -898,5 +900,5 @@
 typeddicts_usage.py:28:17: error[missing-typed-dict-key] Missing required key 'name' in TypedDict `Movie` constructor
 typeddicts_usage.py:28:18: error[invalid-key] Invalid key access on TypedDict `Movie`: Unknown key "title"
 typeddicts_usage.py:40:24: error[invalid-type-form] The special form `typing.TypedDict` is not allowed in type expressions. Did you mean to use a concrete TypedDict or `collections.abc.Mapping[str, object]` instead?
-Found 899 diagnostics
+Found 901 diagnostics
 WARN A fatal error occurred while checking some files. Not all project files were analyzed. See the diagnostics list above for details.

github-actions · 2025-09-27T01:56:55Z

`mypy_primer` results

Changes were detected when running on open source projects

spack (https://github.com/spack/spack)
- lib/spack/spack/vendor/typing_extensions.py:313:12: error[invalid-legacy-type-variable] A legacy `typing.TypeVar` must be immediately assigned to a variable
+ lib/spack/spack/vendor/typing_extensions.py:313:12: error[invalid-legacy-type-variable] A `TypeVar` definition must be a simple variable assignment

setuptools (https://github.com/pypa/setuptools)
- setuptools/_vendor/typing_extensions.py:256:12: error[invalid-legacy-type-variable] A legacy `typing.TypeVar` must be immediately assigned to a variable
+ setuptools/_vendor/typing_extensions.py:256:12: error[invalid-legacy-type-variable] A `TypeVar` definition must be a simple variable assignment
- setuptools/_vendor/typing_extensions.py:1517:27: error[invalid-legacy-type-variable] The `contravariant` parameter of a legacy `typing.TypeVar` cannot have an ambiguous value
- setuptools/_vendor/typing_extensions.py:1517:68: error[invalid-type-form] Variable of type `None` is not allowed in a type expression
+ setuptools/_vendor/typing_extensions.py:1517:48: error[invalid-legacy-type-variable] Starred arguments are not supported in `TypeVar` creation
- Found 800 diagnostics
+ Found 799 diagnostics

scikit-build-core (https://github.com/scikit-build/scikit-build-core)
- src/scikit_build_core/build/_wheelfile.py:51:22: error[no-matching-overload] No overload of function `field` matches arguments
- Found 52 diagnostics
+ Found 51 diagnostics

Memory usage changes were detected when running on open source projects

trio (https://github.com/python-trio/trio)
-     memo fields = ~108MB
+     memo fields = ~103MB

sphinx (https://github.com/sphinx-doc/sphinx)
-     memo metadata = ~42MB
+     memo metadata = ~40MB

prefect (https://github.com/PrefectHQ/prefect)
- TOTAL MEMORY USAGE: ~541MB
+ TOTAL MEMORY USAGE: ~515MB
-     struct metadata = ~30MB
+     struct metadata = ~28MB
-     struct fields = ~35MB
+     struct fields = ~33MB
-     memo fields = ~384MB
+     memo fields = ~366MB

codspeed-hq · 2025-09-27T23:30:46Z

CodSpeed Performance Report

Merging #20598 will improve performances by 5.56%

_{Comparing cjm/legacytv (75eda5e) with main (b086ffe)}

Summary

⚡ 1 improvement
✅ 20 untouched
⏩ 30 skipped¹

Benchmarks breakdown

	Mode	Benchmark	`BASE`	`HEAD`	Change
⚡	WallTime	`small[freqtrade]`	5.2 s	4.9 s	+5.56%

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. ↩

## Summary Typevar attributes (bound/constraints/default) can be either lazily evaluated or eagerly evaluated. Currently they are lazily evaluated for PEP 695 typevars, and eager for legacy and synthetic typevars. #20598 will make them lazy also for legacy typevars, and the ecosystem report on that PR surfaced the issue fixed here (because legacy typevars are much more common in the ecosystem than PEP 695 typevars.) Applying a transform to a typevar (normalization, materialization, or mark-inferable) will reify all lazy attributes and create a new typevar with eager attributes. In terms of Salsa identity, this transformed typevar will be considered different from the original typevar, whether or not the attributes were actually transformed. In general, this is not a problem, since all typevars in a given generic context will be transformed, or not, together. The exception to this was implicit-self vs explicit Self annotations. The typevar we created for implicit self was created initially using inferable typevars, whereas an explicit Self annotation is initially non-inferable, then transformed via mark-inferable when accessed as part of a function signature. If the containing class (which becomes the upper bound of `Self`) is generic, and has e.g. a lazily-evaluated default, then the explicit-Self annotation will reify that default in the upper bound, and the implicit-self would not, leading them to be treated as different typevars, and causing us to fail to solve a call to a method such as `def method(self) -> Self` correctly. The fix here is to treat implicit-self more like explicit-Self, initially creating it as non-inferable and then using the mark-inferable transform on it. This is less efficient, but restores the invariant that all typevars in a given generic context are transformed together, or not, fixing the bug. In the improved-constraint-solver work, the separation of typevars into "inferable" and "non-inferable" is expected to disappear, along with the mark-inferable transform, which would render both this bug and the fix moot. So this fix is really just temporary until that lands. There is a performance regression, but not a huge one: 1-2% on most projects, 5% on one outlier. This seems acceptable, given that it should be fully recovered by removing the mark-inferable transform. ## Test Plan Added mdtests that failed before this change.

MichaReiser · 2025-10-09T06:50:27Z

crates/ty_ide/src/goto_type_definition.rs

-          | ^^^^^
-          |
-        "#);
+        // TODO: This should jump to the definition of `Alias` above.


Can we create an issue for this. It's otherwise hard to remember

I don't think it will be hard to remember, because I have to fix it in order to fix the cases in astral-sh/ty#256 related to TypeAliasType. So in that sense we already have an issue that ensures we won't forget it.

(And I plan to do this PR next.)

AlexWaygood

Only reviewed the tests so far

crates/ty_python_semantic/resources/mdtest/generics/legacy/classes.md

crates/ty_python_semantic/resources/mdtest/generics/legacy/variables.md

crates/ty_python_semantic/src/semantic_index/builder.rs

crates/ty_python_semantic/src/types.rs

crates/ty_python_semantic/src/types/infer/builder.rs

crates/ty_python_semantic/resources/mdtest/generics/legacy/variables.md

AlexWaygood

🚀

crates/ty_python_semantic/src/types/infer/builder.rs

crates/ty_python_semantic/resources/mdtest/generics/legacy/variables.md

crates/ty_python_semantic/resources/mdtest/diagnostics/legacy_typevars.md

Co-authored-by: Alex Waygood <[email protected]>

carljm · 2025-10-09T23:19:43Z

Thanks @AlexWaygood for the excellent review here!

carljm added the ty Multi-file analysis & type inference label Sep 27, 2025

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aidandj mentioned this pull request Sep 27, 2025

Panics for self-referential (recursive) generic classes or type aliases astral-sh/ty#256

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[ty] fix implicit Self on generic class with typevar default #20754

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[ty] Add support for functional TypedDict syntax #20732

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carljm requested review from AlexWaygood, MichaReiser, dcreager and sharkdp as code owners October 9, 2025 00:02

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carljm and others added 9 commits October 9, 2025 14:00

[ty] defer inference of legacy TypeVar bound/constraints/defaults

175b54d

temporary fix for typevar identity with mark_inferable

337987e

Apply suggestions from code review

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Co-authored-by: Alex Waygood <[email protected]>

more review comments

5719863

prefer typing_extensions over setting Python version

e3b9f5a

also error on typing_extensions.TypeVar instantiation in wrong place

b234e94

add typevar-creation diagnostics tests, fix a couple issues

2313684

improve diagnostics

249c0aa

check boolean value of infer_variance kwarg, too

75eda5e

carljm force-pushed the cjm/legacytv branch from f6aaefe to 75eda5e Compare October 9, 2025 21:04

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carljm merged commit 8248193 into main Oct 9, 2025
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dcreager mentioned this pull request Oct 11, 2025

[ty] Add separate type for typevar "identity" #20813

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cgravill mentioned this pull request Oct 13, 2025

[panic] on recursive tuple type astral-sh/ty#1347

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[ty] defer inference of legacy TypeVar bound/constraints/defaults #20598

[ty] defer inference of legacy TypeVar bound/constraints/defaults #20598

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CodSpeed Performance Report

Merging #20598 will improve performances by 5.56%

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Benchmarks breakdown

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