proposal/custom-type-errors.md

@@ -141,7 +141,14 @@ have to think of more syntax and implement it.
 
 ## Design questions (RAE)
 
-1. What happens with Given `TypeError` constraints? Naively, the `TypeError` constraint on an instance would seem to lead to an "inaccessible code" error. (And this point would be right! It *is* inaccessible code.)  *Lennart*:  Yes, we know any methods are inaccessible, but I don't think the compiler needs to know specially about `TypeError`.  Instead the method definition will be treated as usual.  This can always be refined later if we want.
+1. What happens with Given `TypeError` constraints? Naively, the `TypeError` constraint on an instance would seem to lead to an "inaccessible code" error. (And this point would be right! It *is* inaccessible code.)  *Lennart*:  Yes, we know any methods are inaccessible, but I don't think the compiler needs to know specially about `TypeError`.  Instead the method definition will be treated as usual.  This can always be refined later if we want. *RAE:* But I think people will want this:
+
+```
+instanceTypeError"You can't compare functions for equality"=>Eq(a -> b)
+```
+
+>
+> If that's inaccessible code and an error, they can't do this.
 
 1. Relatedly, when definition an instance with a `TypeError` constraint, what should users write in the body? Leaving it empty causes warnings, but anything written in there would never be called.  *Lennart*: See above.