Zonk the returned kind in tcFamTyPats

The motivation is given in Note [tcFamTyPats: zonking the result kind].

Fixes #19250  --  the fix is easy.

(cherry picked from commit 69cab37ae89db16cfd0b734d7fc657e56402a255)

3 files changed, 43 insertions, 0 deletions

diff --git a/compiler/typecheck/TcTyClsDecls.hs b/compiler/typecheck/TcTyClsDecls.hs
index 9617934c13..d0d6b2d561 100644
--- a/compiler/typecheck/TcTyClsDecls.hs
+++ b/compiler/typecheck/TcTyClsDecls.hs
@@ -2747,6 +2747,9 @@ tcFamTyPats fam_tc hs_pats
                                 -- GHC.Rename.Source
                                 tcInferApps typeLevelMode lhs_fun fun_ty hs_pats
 
+       -- Hack alert: see Note [tcFamTyPats: zonking the result kind]
+       ; res_kind <- zonkTcType res_kind
+
        ; traceTc "End tcFamTyPats }" $
          vcat [ ppr fam_tc, text "res_kind:" <+> ppr res_kind ]
 
@@ -2756,6 +2759,33 @@ tcFamTyPats fam_tc hs_pats
     fam_arity = tyConArity fam_tc
     lhs_fun   = noLoc (HsTyVar noExtField NotPromoted (noLoc fam_name))
 
+{- Note [tcFamTyPats: zonking the result kind]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Consider (#19250)
+    F :: forall k. k -> k
+    type instance F (x :: Constraint) = ()
+
+The tricky point is this:
+  is that () an empty type tuple (() :: Type), or
+  an empty constraint tuple (() :: Constraint)?
+We work this out in a hacky way, by looking at the expected kind:
+see Note [Inferring tuple kinds].
+
+In this case, we kind-check the RHS using the kind gotten from the LHS:
+see the call to tcCheckLHsType in tcTyFamInstEqnGuts in GHC.Tc.Tycl.
+
+But we want the kind from the LHS to be /zonked/, so that when
+kind-checking the RHS (tcCheckLHsType) we can "see" what we learned
+from kind-checking the LHS (tcFamTyPats).  In our example above, the
+type of the LHS is just `kappa` (by instantiating the forall k), but
+then we learn (from x::Constraint) that kappa ~ Constraint.  We want
+that info when kind-checking the RHS.
+
+Easy solution: just zonk that return kind.  Of course this won't help
+if there is lots of type-family reduction to do, but it works fine in
+common cases.
+-}
+
 unravelFamInstPats :: TcType -> [TcType]
 -- Decompose fam_app to get the argument patterns
 --
diff --git a/testsuite/tests/polykinds/T19250.hs b/testsuite/tests/polykinds/T19250.hs
new file mode 100644
index 0000000000..6781e4bd39
--- /dev/null
+++ b/testsuite/tests/polykinds/T19250.hs
@@ -0,0 +1,12 @@
+{-# LANGUAGE TypeOperators, TypeFamilies, ConstraintKinds, PolyKinds, DataKinds, EmptyDataDecls #-}
+
+module T19250 where
+
+import Data.Kind
+
+type Exp a = a -> Type
+
+type family Eval (e :: Exp a) :: a
+
+data Collapse :: [Constraint] -> Exp Constraint
+type instance Eval (Collapse '[]) = ()
diff --git a/testsuite/tests/polykinds/all.T b/testsuite/tests/polykinds/all.T
index 6b998f2872..c8d89e0b60 100644
--- a/testsuite/tests/polykinds/all.T
+++ b/testsuite/tests/polykinds/all.T
@@ -216,3 +216,4 @@ test('T16263', normal, compile_fail, [''])
 test('T16902', normal, compile_fail, [''])
 test('CuskFam', normal, compile, [''])
 test('T17841', normal, compile_fail, [''])
+test('T19250', normal, compile, [''])