Expand type/kind synonyms in TyVars before deriving-related typechecking

Before, it was possible to have a datatypes such as

```
type ConstantT a b = a
newtype T (f :: * -> *) (a :: ConstantT * f) = T (f a) deriving Functor

data family TFam (f :: * -> *) (a :: *)
newtype instance TFam f (ConstantT a f) = TFam (f a) deriving Functor
```

fail to eta-reduce because either (1) a TyVar had a kind synonym that
mentioned another TyVar, or (2) an instantiated type was itself a type
synonym that mentioned another TyVar. A little bit of tweaking to
`expandTypeSynonyms` and applying it before the eta-reduction check in
the `deriving` machinery is sufficient to fix this.

Fixes #11416.

Test Plan: ./validate

Reviewers: goldfire, simonpj, austin, bgamari

Reviewed By: simonpj

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1772

GHC Trac Issues: #11416

2 files changed, 20 insertions, 0 deletions

diff --git a/testsuite/tests/deriving/should_compile/T11416.hs b/testsuite/tests/deriving/should_compile/T11416.hs
new file mode 100644
index 0000000000..4696306107
--- /dev/null
+++ b/testsuite/tests/deriving/should_compile/T11416.hs
@@ -0,0 +1,19 @@
+{-# LANGUAGE DeriveFunctor #-}
+{-# LANGUAGE TypeFamilies #-}
+{-# LANGUAGE TypeInType #-}
+module T11416 where
+
+import Data.Kind
+
+type ConstantT a b = a
+
+newtype T f (a :: ConstantT * f) = T (f a)
+  deriving Functor
+
+data family TFam1 (f :: k1) (a :: k2)
+newtype instance TFam1 f (ConstantT a f) = TFam1 (f a)
+  deriving Functor
+
+data family TFam2 (f :: k1) (a :: k2)
+newtype instance TFam2 f (a :: ConstantT * f) = TFam2 (f a)
+  deriving Functor
diff --git a/testsuite/tests/deriving/should_compile/all.T b/testsuite/tests/deriving/should_compile/all.T
index ff26d37d0a..a18a257ff2 100644
--- a/testsuite/tests/deriving/should_compile/all.T
+++ b/testsuite/tests/deriving/should_compile/all.T
@@ -61,3 +61,4 @@ test('T10524', normal, compile, [''])
 test('T11148', normal, run_command,
      ['$MAKE -s --no-print-directory T11148'])
 test('T9968', normal, compile, [''])
+test('T11416', normal, compile, [''])