Fix #14888 by adding more special cases for ArrowT

Summary:
There were previously some situations where `(->)` would
not be desugared or reified as `ArrowT`, leading to various oddities
such as those observed in #14888. We now uniformly treat `(->)` as
`ArrowT` in Template Haskell–world by checking for any tycon that
has the same name as `(->)`, and converting that to `ArrowT`.

Test Plan: make test TEST=T14888

Reviewers: goldfire, bgamari, simonpj

Reviewed By: goldfire, simonpj

Subscribers: simonpj, rwbarton, thomie, carter

GHC Trac Issues: #14888

Differential Revision: https://phabricator.haskell.org/D4466

3 files changed, 31 insertions, 0 deletions

diff --git a/testsuite/tests/th/T14888.hs b/testsuite/tests/th/T14888.hs
new file mode 100644
index 0000000000..e2bcec64ea
--- /dev/null
+++ b/testsuite/tests/th/T14888.hs
@@ -0,0 +1,18 @@
+{-# LANGUAGE TemplateHaskell #-}
+module T14888 where
+
+import Language.Haskell.TH
+
+foo :: $([t| (->) Bool Bool |])
+foo x = x
+
+class Functor' f where
+  fmap' :: (a -> b) -> f a -> f b
+
+instance Functor' ((->) r) where
+  fmap' = (.)
+
+$(return [])
+
+functor'Instances :: String
+functor'Instances = $(reify ''Functor' >>= stringE . pprint)
diff --git a/testsuite/tests/th/T14888.stderr b/testsuite/tests/th/T14888.stderr
new file mode 100644
index 0000000000..963937f9f6
--- /dev/null
+++ b/testsuite/tests/th/T14888.stderr
@@ -0,0 +1,11 @@
+T14888.hs:6:10-30: Splicing type
+    [t| (->) Bool Bool |] ======> Bool -> Bool
+T14888.hs:15:3-11: Splicing declarations return [] ======>
+T14888.hs:18:23-59: Splicing expression
+    reify ''Functor' >>= stringE . pprint
+  ======>
+    "class T14888.Functor' (f_0 :: * -> *)
+    where T14888.fmap' :: forall (f_0 :: * ->
+                                         *) . T14888.Functor' f_0 =>
+                          forall (a_1 :: *) (b_2 :: *) . (a_1 -> b_2) -> f_0 a_1 -> f_0 b_2
+instance T14888.Functor' ((->) r_3 :: * -> *)"
diff --git a/testsuite/tests/th/all.T b/testsuite/tests/th/all.T
index 47e8a9c03c..e9f2838492 100644
--- a/testsuite/tests/th/all.T
+++ b/testsuite/tests/th/all.T
@@ -403,3 +403,5 @@ test('T14838', [], multimod_compile,
      ['T14838.hs', '-v0 -Wincomplete-patterns ' + config.ghc_th_way_flags])
 test('T14817', normal, compile, ['-v0 -ddump-splices -dsuppress-uniques'])
 test('T14843', normal, compile, ['-v0'])
+test('T14888', normal, compile,
+    ['-v0 -ddump-splices -dsuppress-uniques ' + config.ghc_th_way_flags])