Support custom type assignability for complex types

[a-hilaly]

Feature request checklist

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• The change is large enough it can't be addressed with a simple Pull Request
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Title: Support structural type comparison for complex types

Change

Hey folks! In kro we do use CEL to type-check expressions against expected types inferred from OpenAPI specs. Each type gets a path based name based on where it appears in the schema.

Our issue is that CEL compares types by name, not structure. Two types with identical fields but different names are considered incompatible. This causes problems in places like ternary expressions where both branches have the same structure but different type names.

We currently have a AreTypesStructurallyCompatible function that compares types by structure (fields, element types, etc..) rather than names. But this only works outside of CEL's Check() - we use it to validate expression outputs against expected types. And we can't hook into CEL's internal type comparisons, so things like ternary validation fail before we get a chance to run our check.

Would it be technically feasible to expose a hook or option for custom type assignability logic for complex types (lists, maps, structs)? We'd be happy to help with the implementation if you think this would be valuable for CEL.

Example

has(a.spec.items) ? a.spec.items : b.spec.items

Both lists have the same element structure, but CEL rejects this because the type names differ.

Alternatives considered

We tried assigning the same type name to structurally equivalent types (via hashing the structure), but this doesn't work for partially compatible structures. Using dyn() works as a user workaround but loses type safety.

[dmitriplotnikov]

I have stumbled upon something that may be relevant. Kubernetes apiserver seems to be extending cel ref.TypeProvider to support OpenAPI schema: k8s.io/apiserver/pkg/cel/types.go.

[a-hilaly]

Thanks @dmitriplotnikov ! We do actually use it. we use DeclTypeProvider from k8s apiserver - it's how we register our OpenAPI based types with CEL. But the TypeProvider interface only controls type resolution not type assignability. For example the ternary check happens in CEL's checker via internalIsAssignable which compares type names directly and there is no hook to customize the comparison logic.