Fix binder visiblity for default methods

Trac #13998 showed that default methods were getting bogus tyvar
binder visiblity info; and that it matters in the code genreated
by the default-method fill-in mechanism

* The actual fix: in TcTyDecls.mkDefaultMethodType, make TyVarBinders
  with the right visibility info by getting TyConBinders from the
  class TyCon.  (Previously we made up visiblity info, but that
  caused #13998.)

* Define TyCon.tyConTyVarBinders :: [TyConBinder] -> [TyVarBinder]
  which can build correct forall binders for
    a) default methods (Trac #13998)
    b) data constructors
  This was originally BuildTyCl.mkDataConUnivTyVarBinders

* Move mkTyVarBinder, mkTyVarBinders from Type to Var

2 files changed, 44 insertions, 0 deletions

diff --git a/testsuite/tests/deriving/should_compile/T13998.hs b/testsuite/tests/deriving/should_compile/T13998.hs
new file mode 100644
index 0000000000..565d4a35f7
--- /dev/null
+++ b/testsuite/tests/deriving/should_compile/T13998.hs
@@ -0,0 +1,43 @@
+{-# LANGUAGE PolyKinds #-}
+{-# LANGUAGE KindSignatures #-}
+{-# LANGUAGE DataKinds #-}
+{-# LANGUAGE DefaultSignatures #-}
+{-# LANGUAGE GADTs #-}
+
+module T13998 where
+
+import Data.Type.Equality
+
+class EqForall f where
+  eqForall :: f a -> f a -> Bool
+
+class EqForall f => EqForallPoly f where
+  eqForallPoly :: f a -> f b -> Bool
+  default eqForallPoly :: TestEquality f => f a -> f b -> Bool
+  eqForallPoly = defaultEqForallPoly
+
+defaultEqForallPoly :: (TestEquality f, EqForall f) => f a -> f b -> Bool
+defaultEqForallPoly x y = case testEquality x y of
+  Nothing -> False
+  Just Refl -> eqForall x y
+
+
+data Atom = AtomInt | AtomString | AtomBool
+
+data Value (a :: Atom) where
+  ValueInt :: Int -> Value 'AtomInt
+  ValueString :: String -> Value 'AtomString
+  ValueBool :: Bool -> Value 'AtomBool
+
+instance TestEquality Value where
+  testEquality (ValueInt _) (ValueInt _) = Just Refl
+  testEquality (ValueString _) (ValueString _) = Just Refl
+  testEquality (ValueBool _) (ValueBool _) = Just Refl
+  testEquality _ _ = Nothing
+
+instance EqForall Value where
+  eqForall (ValueInt a) (ValueInt b) = a == b
+  eqForall (ValueString a) (ValueString b) = a == b
+  eqForall (ValueBool a) (ValueBool b) = a == b
+
+instance EqForallPoly Value
diff --git a/testsuite/tests/deriving/should_compile/all.T b/testsuite/tests/deriving/should_compile/all.T
index 7c7b29070b..0025d25dee 100644
--- a/testsuite/tests/deriving/should_compile/all.T
+++ b/testsuite/tests/deriving/should_compile/all.T
@@ -93,3 +93,4 @@ test('drv-empty-data', [normalise_errmsg_fun(just_the_deriving)],compile, ['-ddu
 test('drv-phantom', [normalise_errmsg_fun(just_the_deriving)],compile, ['-ddump-deriv -dsuppress-uniques'])
 test('T13813', normal, compile, [''])
 test('T13919', normal, compile, [''])
+test('T13998', normal, compile, [''])