Add a missing zonk (fixes #16902)

In the eager unifier, when unifying (tv1 ~ tv2),
when we decide to swap them over, to unify (tv2 ~ tv1),
I'd forgotten to ensure that tv1's kind was fully zonked,
which is an invariant of uUnfilledTyVar2.

That could lead us to build an infinite kind, or (in the
case of #16902) update the same unification variable twice.

Yikes.

Now we get an error message rather than non-termination,
which is much better.  The error message is not great,
but it's a very strange program, and I can't see an easy way
to improve it, so for now I'm just committing this fix.

Here's the decl
 data F (a :: k) :: (a ~~ k) => Type where
    MkF :: F a

and the rather error message of which I am not proud

  T16902.hs:11:10: error:
    • Expected a type, but found something with kind ‘a1’
    • In the type ‘F a’

3 files changed, 18 insertions, 0 deletions

diff --git a/testsuite/tests/polykinds/T16902.hs b/testsuite/tests/polykinds/T16902.hs
new file mode 100644
index 0000000000..70fc7bd742
--- /dev/null
+++ b/testsuite/tests/polykinds/T16902.hs
@@ -0,0 +1,11 @@
+{-# LANGUAGE DataKinds #-}
+{-# LANGUAGE GADTs #-}
+{-# LANGUAGE PolyKinds #-}
+{-# LANGUAGE TypeOperators #-}
+module Bug where
+
+import Data.Kind
+import Data.Type.Equality
+
+data F (a :: k) :: (a ~~ k) => Type where
+  MkF :: F a
diff --git a/testsuite/tests/polykinds/T16902.stderr b/testsuite/tests/polykinds/T16902.stderr
new file mode 100644
index 0000000000..e265866119
--- /dev/null
+++ b/testsuite/tests/polykinds/T16902.stderr
@@ -0,0 +1,6 @@
+
+T16902.hs:11:10: error:
+    • Expected a type, but found something with kind ‘a1’
+    • In the type ‘F a’
+      In the definition of data constructor ‘MkF’
+      In the data declaration for ‘F’
diff --git a/testsuite/tests/polykinds/all.T b/testsuite/tests/polykinds/all.T
index 5a90ebe723..c67d80d621 100644
--- a/testsuite/tests/polykinds/all.T
+++ b/testsuite/tests/polykinds/all.T
@@ -211,3 +211,4 @@ test('T16221', normal, compile, [''])
 test('T16221a', normal, compile_fail, [''])
 test('T16342', normal, compile, [''])
 test('T16263', normal, compile_fail, [''])
+test('T16902', normal, compile_fail, [''])