Fix newtype eta-reduction

The eta-reduction we do for newype axioms was generating
an inhomogeneous axiom: see #19739.

This patch fixes it in a simple way; see GHC.Tc.TyCl.Build
Note [Newtype eta and homogeneous axioms]

5 files changed, 48 insertions, 0 deletions

diff --git a/testsuite/tests/polykinds/T19739a.hs b/testsuite/tests/polykinds/T19739a.hs
new file mode 100644
index 0000000000..55296b3197
--- /dev/null
+++ b/testsuite/tests/polykinds/T19739a.hs
@@ -0,0 +1,11 @@
+{-# LANGUAGE PolyKinds #-}
+{-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE StandaloneKindSignatures #-}
+module Bug where
+
+import Data.Kind
+import Data.Proxy
+
+type T :: forall (a :: Type) -> Constraint
+class T a where
+  f :: Proxy a
diff --git a/testsuite/tests/polykinds/T19739b.hs b/testsuite/tests/polykinds/T19739b.hs
new file mode 100644
index 0000000000..e0377383da
--- /dev/null
+++ b/testsuite/tests/polykinds/T19739b.hs
@@ -0,0 +1,10 @@
+{-# LANGUAGE PolyKinds #-}
+{-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE StandaloneKindSignatures #-}
+module Bug where
+
+import Data.Kind
+import Data.Proxy
+
+type T :: forall (a :: Type) -> Type
+newtype T a = Mk (Proxy a)
diff --git a/testsuite/tests/polykinds/T19739c.hs b/testsuite/tests/polykinds/T19739c.hs
new file mode 100644
index 0000000000..331d1f74fd
--- /dev/null
+++ b/testsuite/tests/polykinds/T19739c.hs
@@ -0,0 +1,11 @@
+{-# LANGUAGE PolyKinds #-}
+{-# LANGUAGE RankNTypes #-}
+{-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE StandaloneKindSignatures #-}
+module Bug where
+
+import Data.Kind
+import Data.Proxy
+
+type T :: (forall (x :: Type) -> x) -> forall (a :: Type) -> Type
+newtype T f a = MkT (f (Type -> Type) a)
diff --git a/testsuite/tests/polykinds/T19739d.hs b/testsuite/tests/polykinds/T19739d.hs
new file mode 100644
index 0000000000..e316abf620
--- /dev/null
+++ b/testsuite/tests/polykinds/T19739d.hs
@@ -0,0 +1,12 @@
+{-# LANGUAGE PolyKinds #-}
+{-# LANGUAGE RankNTypes #-}
+{-# LANGUAGE StandaloneKindSignatures #-}
+{-# LANGUAGE TypeFamilies #-}
+module Foo where
+
+import Data.Kind
+import Data.Proxy
+
+type F :: Type -> forall (a :: Type) -> Type
+data family F a b
+newtype instance F Int b = MkF (Proxy b)
diff --git a/testsuite/tests/polykinds/all.T b/testsuite/tests/polykinds/all.T
index 581b065fa9..275281f527 100644
--- a/testsuite/tests/polykinds/all.T
+++ b/testsuite/tests/polykinds/all.T
@@ -236,3 +236,7 @@ test('T19093', normal, compile, [''])
 test('T19094', normal, compile, [''])
 test('T19250', normal, compile, [''])
 test('T19522', normal, compile, [''])
+test('T19739a', normal, compile, [''])
+test('T19739b', normal, compile, [''])
+test('T19739c', normal, compile, [''])
+test('T19739d', normal, compile, [''])