| In the current code, the | instance Coercible a b => Coercible (T a) (T b) | is available for both data and newtypes, if T’s type argument has | Representational role, but independent of any constructor presence. See | the note at | https://github.com/nomeata/ghc/compare/ntclass-clean#L9R1902 | for a concise and complete list of the conditions for a Coercible | instance. Right! I explained that badly the first time; thanks for clarifying Joachim. So newtype and data behave alike, except that newtype has the *additional* property that if its constructor is available you can coerce to the representation type. | If I understood Simon’s last suggestion correctly than exporting a type | constructor with a non-Nominal role means “I am fine if you cast this | argument”. If this is not desired (e.g. maybe Ptr a is an example here), | then the library author has to annotate the type argument as Nominal. Yes, that's right. In theory someone could want the coercible instance *plus* the nominal role, or vice versa, but I think we can jump that bridge if we come to it. Simon