casts from json to scalar subtypes don't check constraints

[msullivan]

edgedb> create scalar type x extending str { create constraint expression on (__subject__ not like 'a%') };
OK: CREATE SCALAR TYPE
edgedb> select <x>to_json('"a"');
["a"]
edgedb> select <x><str>to_json('"a"');
edgedb error: ConstraintViolationError: invalid x
  Detail: invalid scalar type 'default::x'

select <x>to_json('"a"') ought to raise an error also.

(I don't think this is specific to json, of course.)
I think the issue is that we are only ever casting to text. We never emit a cast to the real target type.