Delete GenericKind_ in favor of GenericKind_DC

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.

2 files changed, 23 insertions, 0 deletions

diff --git a/testsuite/tests/generics/T21185.hs b/testsuite/tests/generics/T21185.hs
new file mode 100644
index 0000000000..1d2d1f30af
--- /dev/null
+++ b/testsuite/tests/generics/T21185.hs
@@ -0,0 +1,22 @@
+{-# LANGUAGE TypeFamilies #-}
+module T21185 where
+
+import Data.Kind (Type)
+import GHC.Generics (Generic1(..))
+
+type FakeOut x = Int
+
+data D (b :: Type) where
+  MkD :: c -> FakeOut c -> D c
+  deriving Generic1
+
+data family DF (a :: Type)
+data instance DF (b :: Type) where
+  MkDF :: c -> FakeOut c -> DF c
+  deriving Generic1
+
+d :: Rep1 D ()
+d = from1 $ MkD () 42
+
+df :: Rep1 DF ()
+df = from1 $ MkDF () 42
diff --git a/testsuite/tests/generics/all.T b/testsuite/tests/generics/all.T
index 1fccfffc16..a43f585020 100644
--- a/testsuite/tests/generics/all.T
+++ b/testsuite/tests/generics/all.T
@@ -45,3 +45,4 @@ test('T11358', normal, compile_and_run, [''])
 test('T12220', normal, compile, [''])
 test('T15012', [extra_files(['T15012.hs', 'T15012a.hs'])], makefile_test, [])
 test('T19819', normal, compile_and_run, [''])
+test('T21185', normal, compile, [''])