Disallow recursive structs with lifted type parameters #19031
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brianrourkeboll
commented
Oct 25, 2025
| // This type (type instance) has been seen before, so no need to collect the same edges again (and avoid loops!) | ||
| List.exists (typeEquiv g ty) doneTypes | ||
| // This tycon is the outer tycon with a lifted generic argument, e.g., `'a list`. | ||
| || not (isNil doneTypes) && tryTcrefOfAppTy g ty |> ValueOption.bind _.TryDeref |> ValueOption.exists ((===) tycon) |
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We only do this check when doneTypes is not empty because we need to collect the self-referential edge once.
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Separately, I don't think I fully understand when it is safe to access EntityRef.Deref. Tell me if TryDeref is unnecessary here and I will simplify it.
edgarfgp
reviewed
Oct 25, 2025
tests/FSharp.Compiler.ComponentTests/Conformance/Types/RecordTypes/RecordTypes.fs
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I think this PR might also fix #6069 ? |
Hmm yes, probably. I will add a test/tests for mutually-recursive structs. |
edgarfgp
approved these changes
Oct 28, 2025
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Description
Fixes #18993.
Fixes #6069.
I don't know if "lifting" is the correct term for this; let me know if there is a more correct term or phrase that I should use to describe this in the comment and the tests. Maybe nested app ty/type application?
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