Fix non-sendable data seen as sendable in generic recover blocks#4953
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SeanTAllen merged 1 commit intomainfrom Mar 11, 2026
Merged
Fix non-sendable data seen as sendable in generic recover blocks#4953SeanTAllen merged 1 commit intomainfrom
SeanTAllen merged 1 commit intomainfrom
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The sendable() function's TK_ARROW handler used viewpoint_upper() to compute a single bound, then checked sendability of that bound. For generic arrow types like this->(T #any !), viewpoint_upper treats this as box, and cap_view_upper(box, #alias) yields tag — which is sendable. But concrete instantiations like ref->(T #any !) = T #alias are NOT sendable. Replace the single-bound computation with case-splitting: enumerate the concrete capabilities the left side could be (matching the sets used by viewpoint_reifythis and viewpoint_reifytypeparam for subtype checking) and check that ALL produce sendable types. Concrete-cap arrow types still use viewpoint_upper directly, preserving the #4244 fix. Closes #4459 Design: #4949
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Consider moving the generic->concrete code to a helper function that could be reused, but doesn't matter that much.
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The compiler's
sendable()function incorrectly classified generic arrow types likethis->(T #any !)as sendable.viewpoint_uppertreatsthisasbox, andcap_view_upper(box, #alias)yieldstag— which is sendable. But concrete instantiations likeref->(T #any !) = T #aliasare not sendable, allowing unsound recovery of non-sendable data.The fix replaces the single-bound computation in
sendable()'sTK_ARROWhandler with case-splitting: enumerate the concrete capabilities the left side could be (matching the sets used byviewpoint_reifythisandviewpoint_reifytypeparamfor subtype checking) and check that ALL produce sendable types. Concrete-cap arrow types still useviewpoint_upperdirectly, preserving the #4244 fix.Filed #4952 as follow-up for
immutable_or_opaque()which may have an analogous issue.Closes #4459
Design: #4949