Create di_scoped_tvs for associated data family instances properly

See `Note [Associated data family instances and di_scoped_tvs]` in
`GHC.Tc.TyCl.Instance`, which explains all of the moving parts.

Fixes #18055.

2 files changed, 42 insertions, 0 deletions

diff --git a/testsuite/tests/deriving/should_compile/T18055.hs b/testsuite/tests/deriving/should_compile/T18055.hs
new file mode 100644
index 0000000000..35f3750840
--- /dev/null
+++ b/testsuite/tests/deriving/should_compile/T18055.hs
@@ -0,0 +1,41 @@
+{-# LANGUAGE DeriveAnyClass        #-}
+{-# LANGUAGE DerivingVia           #-}
+{-# LANGUAGE MultiParamTypeClasses #-}
+{-# LANGUAGE TypeFamilies          #-}
+module Bug where
+
+import Data.Kind
+
+-----
+
+data Block m = Block
+
+class NoThunks m where
+
+newtype AllowThunk a = AllowThunk a
+
+class GetHeader blk where
+  data family Header blk :: Type
+
+instance GetHeader (Block m) where
+  newtype Header (Block m) = BlockHeader { main :: Header m }
+    deriving NoThunks via AllowThunk (Header (Block m))
+
+-----
+
+class C a where
+  data D a
+
+class X a b
+
+instance C (Maybe a) where
+  data D (Maybe a) deriving (X a)
+
+instance C [a] where
+  newtype D [a] = MkDList Bool
+
+newtype MyList a = MkMyList [a]
+
+instance C (MyList a) where
+  newtype D (MyList a) = MkDMyList Bool
+    deriving (X a) via D [a]
diff --git a/testsuite/tests/deriving/should_compile/all.T b/testsuite/tests/deriving/should_compile/all.T
index a00617b057..8a363e72f9 100644
--- a/testsuite/tests/deriving/should_compile/all.T
+++ b/testsuite/tests/deriving/should_compile/all.T
@@ -123,3 +123,4 @@ test('T17324', normal, compile, [''])
 test('T17339', normal, compile,
      ['-ddump-simpl -dsuppress-idinfo -dno-typeable-binds'])
 test('T17880', normal, compile, [''])
+test('T18055', normal, compile, [''])