Note [Requirements for deriving Generic and Rep]

In the following, T, Tfun, and Targ are "meta-variables" ranging over type
expressions.

(Generic T) and (Rep T) are derivable for some type expression T if the
following constraints are satisfied.

  (a) D is a type constructor *value*. In other words, D is either a type
      constructor or it is equivalent to the head of a data family instance (up to
      alpha-renaming).

  (b) D cannot have a "stupid context".
      See Note [The stupid context] in GHC.Core.DataCon.

  (c) The right-hand side of D cannot include existential types, universally
      quantified types, or "exotic" unlifted types. An exotic unlifted type
      is one which is not listed in the definition of allowedUnliftedTy
      (i.e., one for which we have no representation type).
      See Note [Generics and unlifted types]

  (d) T :: *.

(Generic1 T) and (Rep1 T) are derivable for some type expression T if the
following constraints are satisfied.

  (a),(b),(c) As above.

  (d) T must expect arguments, and its last parameter must have kind *.

      We use `a' to denote the parameter of D that corresponds to the last
      parameter of T.

  (e) For any type-level application (Tfun Targ) in the right-hand side of D
      where the head of Tfun is not a tuple constructor:

      (b1) `a' must not occur in Tfun.

      (b2) If `a' occurs in Targ, then Tfun :: * -> *.