Note [Inferring the instance context]

There are two sorts of 'deriving', as represented by the two constructors
for DerivContext:

  * InferContext mb_wildcard: This can either be:
    - The deriving clause for a data type.
        (e.g, data T a = T1 a deriving( Eq ))
      In this case, mb_wildcard = Nothing.
    - A standalone declaration with an extra-constraints wildcard
        (e.g., deriving instance _ => Eq (Foo a))
      In this case, mb_wildcard = Just loc, where loc is the location
      of the extra-constraints wildcard.

    Here we must infer an instance context,
    and generate instance declaration
      instance Eq a => Eq (T a) where ...

  * SupplyContext theta: standalone deriving
      deriving instance Eq a => Eq (T a)
    Here we only need to fill in the bindings;
    the instance context (theta) is user-supplied

For the InferContext case, we must figure out the
instance context (inferConstraintsStock). Suppose we are inferring
the instance context for
    C t1 .. tn (T s1 .. sm)
There are two cases

  * (T s1 .. sm) :: *         (the normal case)
    Then we behave like Eq and guess (C t1 .. tn t)
    for each data constructor arg of type t.  More
    details below.

  * (T s1 .. sm) :: * -> *    (the functor-like case)
    Then we behave like Functor.

In both cases we produce a bunch of un-simplified constraints
and them simplify them in simplifyInstanceContexts; see
Note [Simplifying the instance context].

In the functor-like case, we may need to unify some kind variables with * in
order for the generated instance to be well-kinded. An example from #10524:

  newtype Compose (f :: k2 -> *) (g :: k1 -> k2) (a :: k1)
    = Compose (f (g a)) deriving Functor

Earlier in the deriving pipeline, GHC unifies the kind of Compose f g
(k1 -> *) with the kind of Functor's argument (* -> *), so k1 := *. But this
alone isn't enough, since k2 wasn't unified with *:

  instance (Functor (f :: k2 -> *), Functor (g :: * -> k2)) =>
    Functor (Compose f g) where ...

The two Functor constraints are ill-kinded. To ensure this doesn't happen, we:

  1. Collect all of a datatype's subtypes which require functor-like
     constraints.
  2. For each subtype, create a substitution by unifying the subtype's kind
     with (* -> *).
  3. Compose all the substitutions into one, then apply that substitution to
     all of the in-scope type variables and the instance types.