# let test =
let module Local = struct type t = Foo end
in Local.Foo;;
Error: This `let module' expression has type Local.t
In this type, the locally bound module name Local escapes its scope
However, exceptions do not create a local type, they instead augment the global open-ended "exn" type, so the type-checker won't detect the leak.
Finally, you don't want to disallow local exception declarations, as they're very useful in various situations -- simply moving an existing module to a local scope.
(I agree that the type regression when escaping is quite weird and should be avoided if possible.)