Compare FunTys as if they were TyConApps.

See Note [Equality on FunTys] in TyCoRep.

Close #17675.
Close #17655, about documentation improvements included in
this patch.
Close #19677, about a further mistake around FunTy.

test cases: typecheck/should_compile/T19677

2 files changed, 20 insertions, 0 deletions

diff --git a/testsuite/tests/typecheck/should_compile/T19677.hs b/testsuite/tests/typecheck/should_compile/T19677.hs
new file mode 100644
index 0000000000..1b29beb0d1
--- /dev/null
+++ b/testsuite/tests/typecheck/should_compile/T19677.hs
@@ -0,0 +1,19 @@
+{-# LANGUAGE ExplicitForAll, PolyKinds, DataKinds, TypeFamilies, GADTs #-}
+
+module T19677 where
+
+import GHC.Exts
+
+class C x where
+  meth :: x -> ()
+instance C (a -> b) where
+  meth _ = ()
+
+type family Lifty throttle where
+  Lifty Int = LiftedRep
+
+data G a where
+  MkG :: G Int
+
+foo :: forall i (a :: TYPE (Lifty i)) (b :: TYPE (Lifty i)). G i -> (a -> b) -> ()
+foo MkG = meth
diff --git a/testsuite/tests/typecheck/should_compile/all.T b/testsuite/tests/typecheck/should_compile/all.T
index b6735408db..9983ff20da 100644
--- a/testsuite/tests/typecheck/should_compile/all.T
+++ b/testsuite/tests/typecheck/should_compile/all.T
@@ -739,6 +739,7 @@ test('T18412', normal, compile, [''])
 test('T18470', normal, compile, [''])
 test('T18308', normal, compile, [''])
 test('T18323', normal, compile, [''])
+test('T19677', normal, compile, [''])
 test('T18585', normal, compile, [''])
 test('T18831', normal, compile, [''])
 test('T18920', normal, compile, [''])