Note [Synonyms implement abstract data]

An abstract data type or class can be implemented using a type synonym,
but ONLY if:

  1. T, as a standalone occurrence, is a valid type
     (T is "curryable"), and

  2. T is valid in an instance head.

This gives rise to the following conditions under which we can implement
an abstract data declaration @data T@ using a type synonym @type T tvs = rhs@:

  1. The type synonym T is nullary (tvs is null).

  2. The rhs must not contain any foralls, quantified types, or type family
     applications.
     See 'invalidAbsDataSubTypes' which computes a collection of
     invalid subtypes.

See also 'HowAbstract' and Note [Skolem abstract data].