Should `after` hooks be allowed to mutate the value? about spec

[moredip]

I think another con would be it would make type signatures really fiddly, and/or we'd lose a lot of type safety.

[beeme1mr]

We could allow the after hook to mutate the value but require that the type signature itself doesn't change.

[toddbaert]

In addition to side-effects that don't impact the returned value at all (ending an OTel span, logging, etc), one of the use-cases we have for this is validation of a resolved flag value. However, that use-case is resolved simply by throwing in the after hook if the retrieved value is invalid, which results in the evaluation defaulting, which is probably exactly what we'd want.

That is to say, I think we can, for now, close this since both those basic use-cases are satisfied. I don't think we need to support mutation of the after value, for now. Please re-open if you disagree!