What if we just reject cast functions with a non-null prosqlbody?
Given the interaction with array and domain coercion, we need to think more about error-safe type casts for user-defined range, multirange, and composite types. For now, we should disallow error-safe casting for these types.
I agree. It's far better to get this working for the majority of cases, and we can chip away at more obscure types later. There will *always* be user defined types that we cannot account for.
