Effect on API resilience Opaque result types are part of the result type of a function/type of a variable/element type of a subscript, which cannot be changed without affecting API resilience. We could allow an API originally specified using an opaque result type to later evolve to specify the specific result type. The result type itself would have to become visible to clients, and this might affect source compatibility, but (mangled name aside) such a change would be resilient.