[ty] Infer variance through recursive protocols - #28077
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This would help fix a user-reported issue in astral-sh/ty#4407 |
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I'll revive it, thanks! |
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Typing conformance resultsNo changes detected ✅Current numbersThe percentage of diagnostics emitted that were expected errors held steady at 97.79%. The percentage of expected errors that received a diagnostic held steady at 94.33%. The number of fully passing files held steady at 112/136. |
Memory usage reportSummary
Significant changesClick to expand detailed breakdownprefect
trio
sphinx
flake8
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| Lint rule | Added | Removed | Changed |
|---|---|---|---|
invalid-protocol |
3 | 0 | 0 |
| Total | 3 | 0 | 0 |
Flaky changes detected. This PR summary excludes flaky changes; see the HTML report for details.
Raw diff:
antidote (https://github.com/Finistere/antidote)
+ src/antidote/core/__init__.py:1300:7 error[invalid-protocol] Type variable `T` in protocol `InjectedMethod` should be covariant, but is invariant
comtypes (https://github.com/enthought/comtypes)
+ comtypes/hints.pyi:236:7 error[invalid-protocol] Type variable `_R_Get` in protocol `_Descriptor` should be covariant, but is invariant
+ comtypes/hints.pyi:236:7 error[invalid-protocol] Type variable `_T_Inst` in protocol `_Descriptor` should be contravariant, but is invariant
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charliermarsh
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carljm
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dhruvmanila
August 28, 2026 01:13
charliermarsh
marked this pull request as ready for review
August 28, 2026 01:28
Merging this PR will improve performance by 5.07%
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| Mode | Benchmark | BASE |
HEAD |
Efficiency | |
|---|---|---|---|---|---|
| ⚡ | Simulation | ty_micro[inherited_recursive_protocol_constructor] |
10.4 s | 9.8 s | +5.1% |
| ⚡ | Simulation | ty_micro[inherited_recursive_protocol_diagnostic] |
10.4 s | 9.9 s | +5.04% |
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Curious why performance improved? Use the CodSpeed MCP and ask your agent.
Comparing charlie/recursive-protocol-variance (de54cd1) with main (1776127)
Footnotes
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84 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. ↩
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Summary
We now infer and validate variance through direct, mutual, and expanding recursive protocol references. A recursive member no longer suppresses the variance mismatch in this protocol:
We separate constructing variance equations from evaluating them. Recursive references name another parameter’s equation instead of expanding its definition, so even references like
P[list[T]]produce a finite problem.To validate a declaration, we solve mutually dependent parameters together, starting at bivariance and updating until the results stop changing. This prevents a protocol’s declared variance from serving as evidence for its own correctness. Declarations outside that recursive group remain authoritative. Dependencies also respect argument erasure: an alias that ignores its type argument does not create a recursive dependency through that argument. Ordinary type inference continues to honor declared variance, including for unused parameters.
The symbolic terms and fixed-point iteration follow the general approach described in the Rust compiler’s variance-inference documentation, adapted here to validating Python protocol declarations.
This also enables structural variance inference for recursive PEP 695 protocols, including invariance from writable attributes. Recursive type aliases and descriptor setters whose accepted values cannot be represented by a single type remain deferred.