[ty] Emit diagnostic when a type variable with a default is followed by one without a default

[11happy]

Summary

This PR fixes astral-sh/ty#1651

Test Plan

I have added mdtest

[astral-sh-bot]

Diagnostic diff on typing conformance tests

Changes were detected when running ty on typing conformance tests

--- old-output.txt	2025-12-14 19:31:44.194785139 +0000
+++ new-output.txt	2025-12-14 19:31:47.826806321 +0000
@@ -439,6 +439,7 @@
 generics_basic.py:171:1: error[invalid-generic-class] `Generic` base class must include all type variables used in other base classes
 generics_basic.py:172:1: error[invalid-generic-class] `Generic` base class must include all type variables used in other base classes
 generics_basic.py:199:5: error[type-assertion-failure] Type `Iterator[Any]` does not match asserted type `Unknown`
+generics_defaults.py:24:40: error[invalid-generic-class] Type parameter `T` without a default cannot follow earlier parameter `DefaultStrT` with a default
 generics_defaults.py:30:1: error[type-assertion-failure] Type `type[NoNonDefaults[str, int]]` does not match asserted type `<class 'NoNonDefaults'>`
 generics_defaults.py:31:1: error[type-assertion-failure] Type `type[NoNonDefaults[str, int]]` does not match asserted type `<class 'NoNonDefaults[str, int]'>`
 generics_defaults.py:32:1: error[type-assertion-failure] Type `type[NoNonDefaults[str, int]]` does not match asserted type `<class 'NoNonDefaults[str, int]'>`
@@ -1025,4 +1026,4 @@
 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] Unknown key "title" for 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
-Found 1027 diagnostics
+Found 1028 diagnostics

[astral-sh-bot]

mypy_primer results

Changes were detected when running on open source projects

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 41 diagnostics
+ Found 42 diagnostics

pydantic (https://github.com/pydantic/pydantic)
- pydantic/fields.py:943:5: error[invalid-parameter-default] Default value of type `PydanticUndefinedType` is not assignable to annotated parameter type `dict[str, Divergent] | ((dict[str, Divergent], /) -> None) | None`
+ pydantic/fields.py:943:5: error[invalid-parameter-default] Default value of type `PydanticUndefinedType` is not assignable to annotated parameter type `dict[str, int | float | str | ... omitted 3 union elements] | ((dict[str, int | float | str | ... omitted 3 union elements], /) -> None) | None`
- pydantic/fields.py:983:5: error[invalid-parameter-default] Default value of type `PydanticUndefinedType` is not assignable to annotated parameter type `dict[str, Divergent] | ((dict[str, Divergent], /) -> None) | None`
+ pydantic/fields.py:983:5: error[invalid-parameter-default] Default value of type `PydanticUndefinedType` is not assignable to annotated parameter type `dict[str, int | float | str | ... omitted 3 union elements] | ((dict[str, int | float | str | ... omitted 3 union elements], /) -> None) | None`
- pydantic/fields.py:1026:5: error[invalid-parameter-default] Default value of type `PydanticUndefinedType` is not assignable to annotated parameter type `dict[str, Divergent] | ((dict[str, Divergent], /) -> None) | None`
+ pydantic/fields.py:1026:5: error[invalid-parameter-default] Default value of type `PydanticUndefinedType` is not assignable to annotated parameter type `dict[str, int | float | str | ... omitted 3 union elements] | ((dict[str, int | float | str | ... omitted 3 union elements], /) -> None) | None`
- pydantic/fields.py:1066:5: error[invalid-parameter-default] Default value of type `PydanticUndefinedType` is not assignable to annotated parameter type `dict[str, Divergent] | ((dict[str, Divergent], /) -> None) | None`
+ pydantic/fields.py:1066:5: error[invalid-parameter-default] Default value of type `PydanticUndefinedType` is not assignable to annotated parameter type `dict[str, int | float | str | ... omitted 3 union elements] | ((dict[str, int | float | str | ... omitted 3 union elements], /) -> None) | None`
- pydantic/fields.py:1109:5: error[invalid-parameter-default] Default value of type `PydanticUndefinedType` is not assignable to annotated parameter type `dict[str, Divergent] | ((dict[str, Divergent], /) -> None) | None`
+ pydantic/fields.py:1109:5: error[invalid-parameter-default] Default value of type `PydanticUndefinedType` is not assignable to annotated parameter type `dict[str, int | float | str | ... omitted 3 union elements] | ((dict[str, int | float | str | ... omitted 3 union elements], /) -> None) | None`
- pydantic/fields.py:1148:5: error[invalid-parameter-default] Default value of type `PydanticUndefinedType` is not assignable to annotated parameter type `dict[str, Divergent] | ((dict[str, Divergent], /) -> None) | None`
+ pydantic/fields.py:1148:5: error[invalid-parameter-default] Default value of type `PydanticUndefinedType` is not assignable to annotated parameter type `dict[str, int | float | str | ... omitted 3 union elements] | ((dict[str, int | float | str | ... omitted 3 union elements], /) -> None) | None`
- pydantic/fields.py:1188:5: error[invalid-parameter-default] Default value of type `PydanticUndefinedType` is not assignable to annotated parameter type `dict[str, Divergent] | ((dict[str, Divergent], /) -> None) | None`
+ pydantic/fields.py:1188:5: error[invalid-parameter-default] Default value of type `PydanticUndefinedType` is not assignable to annotated parameter type `dict[str, int | float | str | ... omitted 3 union elements] | ((dict[str, int | float | str | ... omitted 3 union elements], /) -> None) | None`
- pydantic/fields.py:1567:13: error[invalid-argument-type] Argument is incorrect: Expected `dict[str, Divergent] | ((dict[str, Divergent], /) -> None) | None`, found `Top[dict[Unknown, Unknown]] | (((dict[str, Divergent], /) -> None) & ~Top[dict[Unknown, Unknown]]) | None`
+ pydantic/fields.py:1567:13: error[invalid-argument-type] Argument is incorrect: Expected `dict[str, int | float | str | ... omitted 3 union elements] | ((dict[str, int | float | str | ... omitted 3 union elements], /) -> None) | None`, found `Top[dict[Unknown, Unknown]] | (((dict[str, int | float | str | ... omitted 3 union elements], /) -> None) & ~Top[dict[Unknown, Unknown]]) | None`

No memory usage changes detected ✅

[carljm]

Thanks for the PR!

This is emitting two new diagnostics on the conformance suite. The one in generics_defaults.py is correct: we should emit this diagnostic there.

The one at https://github.com/python/typing/blob/main/conformance/tests/generics_defaults_specialization.py#L42 looks wrong -- there should not be any diagnostic there, and the one we emit is clearly not right, since it references T1, a type variable from the superclass which we are specializing away with int. And it claims T1 is following some other type variable.

I haven't looked at the code here yet, but it looks like in that subclassing scenario we are checking the wrong set of type variables somehow.

There are a lot of new diagnostics in the ecosystem here. I would recommend first fixing the above issue, and then seeing if the ecosystem impact is reduced.

[11happy]

Thanks for the PR!

This is emitting two new diagnostics on the conformance suite. The one in generics_defaults.py is correct: we should emit this diagnostic there.

The one at https://github.com/python/typing/blob/main/conformance/tests/generics_defaults_specialization.py#L42 looks wrong -- there should not be any diagnostic there, and the one we emit is clearly not right, since it references T1, a type variable from the superclass which we are specializing away with int. And it claims T1 is following some other type variable.

I haven't looked at the code here yet, but it looks like in that subclassing scenario we are checking the wrong set of type variables somehow.

There are a lot of new diagnostics in the ecosystem here. I would recommend first fixing the above issue, and then seeing if the ecosystem impact is reduced.

Thank you for the pointers! , I have updated the approach & added the new test as well.
Thank you : )

[11happy]

ping! have resolved the merged conflict
would appreciate a review
Thank you : )

[AlexWaygood]

thank you

[codspeed-hq]

CodSpeed Performance Report

Merging #21787 will degrade performances by 5.02%

_{Comparing 11happy:ty_typevar_invalid_order (ceea5c5) with main (8bc753b)}

Summary

❌ 1 regression
✅ 21 untouched
⏩ 30 skipped¹

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Mode	Benchmark	BASE	HEAD	Change
❌	WallTime	large[pydantic]	201.8 s	212.5 s	-5.02%

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. ↩