Ryan Scott authored Mar 06, 2022 and Marge Bot committed Mar 07, 2022

When deriving a `Generic1` instance, we need to know what the last type
variable of a data type is. Previously, there were two mechanisms to determine
this information:

* `GenericKind_`, where `Gen1_` stored the last type variable of a data type
   constructor (i.e., the `tyConTyVars`).
* `GenericKind_DC`, where `Gen1_DC` stored the last universally quantified
  type variable in a data constructor (i.e., the `dataConUnivTyVars`).

These had different use cases, as `GenericKind_` was used for generating
`Rep(1)` instances, while `GenericKind_DC` was used for generating `from(1)`
and `to(1)` implementations. This was already a bit confusing, but things went
from confusing to outright wrong after !6976. This is because after !6976,
the `deriving` machinery stopped using `tyConTyVars` in favor of
`dataConUnivTyVars`. Well, everywhere with the sole exception of
`GenericKind_`, which still continued to use `tyConTyVars`. This lead to
disaster when deriving a `Generic1` instance for a GADT family instance, as
the `tyConTyVars` do not match the `dataConUnivTyVars`. (See #21185.)

The fix is to stop using `GenericKind_` and replace it with `GenericKind_DC`.
For the most part, this proves relatively straightforward. Some highlights:

* The `forgetArgVar` function was deleted entirely, as it no longer proved
  necessary after `GenericKind_`'s demise.
* The substitution that maps from the last type variable to `Any` (see
  `Note [Generating a correctly typed Rep instance]`) had to be moved from
  `tc_mkRepTy` to `tc_mkRepFamInsts`, as `tc_mkRepTy` no longer has access to
  the last type variable.

Fixes #21185.