[ty] Ensure a type[] type T is always considered assignable to a union that holds T and a protocol P#23901
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AlexWaygood merged 2 commits intomainfrom Mar 12, 2026
Merged
[ty] Ensure a type[] type T is always considered assignable to a union that holds T and a protocol P#23901AlexWaygood merged 2 commits intomainfrom
type[] type T is always considered assignable to a union that holds T and a protocol P#23901AlexWaygood merged 2 commits intomainfrom
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Typing conformance resultsNo changes detected ✅Current numbersThe percentage of diagnostics emitted that were expected errors held steady at 85.29%. The percentage of expected errors that received a diagnostic held steady at 78.13%. The number of fully passing files held steady at 64/132. |
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Memory usage reportMemory usage unchanged ✅ |
dcreager
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Mar 12, 2026
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| /// Allow unsound upcasts to callables, such as treating `type[]` as | ||
| /// `Callable[<constructor signature>, <instance of class>]`. |
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No concerns with the approach or the implementation. But this describes how some upcasts are unsound, but doesn't describe why or when we want those. Do we have a good handle on that that we could persist as a comment here? (If that would take too much analysis I'm fine punting on that, though it might still deserve a TODO comment to figure out whether any existing unsound uses could migrate to sound)
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Thank you! I agree the docs weren't great. I spruced 'em up a bit
…union that holds `T` and a protocol `P`
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Summary
Fixes astral-sh/ty#2981. Since the bug exists for both protocols and Callables, I pulled out the special-case logic inserted into
relation.rsin #23834 intoType::try_upcast_to_callable()itself. But there are many uses of that method across the codebase, and nearly all uses do not care about unsound upcasts, so rather than forcing all callers to pass apolicyargument totry_upcast_to_callable(), I instead added a new method,try_upcast_to_callable_with_policy, which is now called byType::try_upcast_to_callable.Test Plan
mdtests extended