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[mypyc] Reject unknown bases for native classes - #22204
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Fixes mypyc/mypyc#1200. Reject native classes with an `Any` base, since their instance layout is not known during compilation. Suggest using `@mypyc_attr(native_class=False)` instead. Preserve the original base expressions for non-native classes with unknown bases so Python determines their inheritance and instance layout at runtime.
Extract the unknown-base check into a helper and include the relevant base name in the diagnostic. Resolve known type aliases before selecting the unknown base to report. Extend the existing runtime and error cases with known and unknown bases in both orders, two unknown bases, and an alias of a known base. Explicitly check inherited attributes, methods, and initializers on direct and transitive subclasses.
The base class handling is shared with other generic non-native classes: Any bases now use the same path that evaluates base expressions as written, resolves __mro_entries__ and sets __orig_bases__. Extend the unknown-base test with Generic[T] in both base orders and generic metadata checks. Also cover a subscripted compiled generic base (KnownGeneric[int]) next to an Any base, which used to fail at import with "type 'KnownGeneric' is not subscriptable". Also narrow the unknown-base diagnostic guard to expression types that declare a name.
p-sawicki
approved these changes
Oct 14, 2026
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Fixes mypyc/mypyc#1200.
Reject native classes with an
Anybase, since their instance layout isnot known during compilation. Suggest using
@mypyc_attr(native_class=False)instead.
Preserve the original base expressions for non-native classes with unknown
bases so Python determines their inheritance and instance layout at runtime.