Make the unifier a fixpoint even for the free kind vars of a tyvar

The (pure) unifier tcUnifyTys returns an idempotent substitution.
But previously the kinds of type variables free in the range of
the subst could have un-substituted kind variables.

This patch fixes that, fixing Trac #9106.

See Note [Finding the substitution fixpoint] in Unify

3 files changed, 23 insertions, 0 deletions

diff --git a/testsuite/tests/polykinds/T9106.hs b/testsuite/tests/polykinds/T9106.hs
new file mode 100644
index 0000000000..eaf0364235
--- /dev/null
+++ b/testsuite/tests/polykinds/T9106.hs
@@ -0,0 +1,14 @@
+{-# LANGUAGE MultiParamTypeClasses, DataKinds, FunctionalDependencies,
+   KindSignatures, PolyKinds, FlexibleInstances, FlexibleContexts,
+   UndecidableInstances #-}
+
+module T9106 where
+
+import GHC.TypeLits
+
+class FunctorN (n :: Nat) f (a :: *) (fa :: *) | n f a -> fa where
+
+instance FunctorN 0 f a a where
+
+instance FunctorN n f a (f fa)
+
diff --git a/testsuite/tests/polykinds/T9106.stderr b/testsuite/tests/polykinds/T9106.stderr
new file mode 100644
index 0000000000..0b239f2ea4
--- /dev/null
+++ b/testsuite/tests/polykinds/T9106.stderr
@@ -0,0 +1,8 @@
+
+T9106.hs:13:10:
+    Illegal instance declaration for ‘FunctorN n f a (f fa)’
+      The liberal coverage condition fails in class ‘FunctorN’
+        for functional dependency: ‘n f a -> fa’
+      Reason: lhs types ‘n’, ‘f’, ‘a’
+        do not jointly determine rhs type ‘f fa’
+    In the instance declaration for ‘FunctorN n f a (f fa)’
diff --git a/testsuite/tests/polykinds/all.T b/testsuite/tests/polykinds/all.T
index 3634d83537..96faa6725b 100644
--- a/testsuite/tests/polykinds/all.T
+++ b/testsuite/tests/polykinds/all.T
@@ -100,3 +100,4 @@ test('T8566a', expect_broken(8566), compile,[''])
 test('T7481', normal, compile_fail,[''])
 test('T8705', normal, compile, [''])
 test('T8985', normal, compile, [''])
+test('T9106', normal, compile_fail, [''])