solve generic function converters against the field type#4158
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shobhitmehro wants to merge 1 commit into
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solve generic function converters against the field type#4158shobhitmehro wants to merge 1 commit into
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Summary: Goal: A generic function used as an attrs converter, such as copy.deepcopy, no longer produces a spurious error when you construct the class. Background: attrs lets a field declare a converter that transforms the constructor argument before it is stored, and pyrefly types the generated `__init__` parameter as the converter's input type. When the converter is a generic function like, whose type is `(_T) -> _T`, that input is a bare type variable `_T` that escapes its function scope, so every argument was rejected with a message about a parameter of type `_T`. How: We add a parallel arm that recognizes a generic function converter and solves it the same way the class path does, by instantiating the function with fresh variables, constraining its return type against the declared field type, and reading back the solved input as the `__init__` parameter. A variable that cannot be solved is finalized to a gradual type so uncertain cases fall back instead of erroring. fixes facebook#4155 Differential Revision: D112165450
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@shobhitmehro has exported this pull request. If you are a Meta employee, you can view the originating Diff in D112165450. |
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According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅ |
yangdanny97
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Jul 16, 2026
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This pull request has been merged in e8017ac. |
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Summary:
Goal: A generic function used as an attrs converter, such as copy.deepcopy, no longer produces a spurious error when you construct the class.
Background: attrs lets a field declare a converter that transforms the constructor argument before it is stored, and pyrefly types the generated
__init__parameter as the converter's input type. When the converter is a generic function like, whose type is(_T) -> _T, that input is a bare type variable_Tthat escapes its function scope, so every argument was rejected with a message about a parameter of type_T.How: We add a parallel arm that recognizes a generic function converter and solves it the same way the class path does, by instantiating the function with fresh variables, constraining its return type against the declared field type, and reading back the solved input as the
__init__parameter. A variable that cannot be solved is finalized to a gradual type so uncertain cases fall back instead of erroring.fixes #4155
Differential Revision: D112165450