Fix #13885 by freshening reified GADT constructors' universal tyvars

Summary:
When reifying GADTs with Template Haskell, the universally quantified
type variables were being reused across both the data type head and the
constructors' type signatures. This had the annoying effect of causing sets
of differently scoped variables to have the same uniques. To avoid this, we
now freshen the universal tyvars before reifying the constructors so as to
ensure they have distinct uniques.

Test Plan: make test TEST=T13885

Reviewers: goldfire, austin, bgamari, simonpj

Reviewed By: simonpj

Subscribers: rwbarton, thomie

GHC Trac Issues: #13885

Differential Revision: https://phabricator.haskell.org/D3867

1 files changed, 23 insertions, 0 deletions

diff --git a/testsuite/tests/th/T13885.hs b/testsuite/tests/th/T13885.hs
new file mode 100644
index 0000000000..0e29c88610
--- /dev/null
+++ b/testsuite/tests/th/T13885.hs
@@ -0,0 +1,23 @@
+{-# LANGUAGE ExistentialQuantification #-}
+{-# LANGUAGE TemplateHaskell #-}
+{-# LANGUAGE TypeFamilies #-}
+{-# LANGUAGE TypeOperators #-}
+module Main where
+
+import Data.Function (on)
+import Language.Haskell.TH.Syntax
+
+data a :~: b = a ~ b => Refl
+
+$(return [])
+
+main :: IO ()
+main = print
+  $(do TyConI (DataD _ _ tycon_tyvars _
+                     [ForallC con_tyvars _ _] _) <- reify ''(:~:)
+
+       let tvbName :: TyVarBndr -> Name
+           tvbName (PlainTV  n)   = n
+           tvbName (KindedTV n _) = n
+
+       lift $ and $ zipWith ((/=) `on` tvbName) tycon_tyvars con_tyvars)