Fix two ASSERT buglets in reifyDataCon

Two `ASSERT`s in `reifyDataCon` were always using `arg_tys`, but
`arg_tys` is not meaningful for GADT constructors. In fact, it's
worse than non-meaningful, since using `arg_tys` when reifying a
GADT constructor can lead to failed `ASSERT`ions, as #17305
demonstrates.

This patch applies the simplest possible fix to the immediate
problem. The `ASSERT`s now use `r_arg_tys` instead of `arg_tys`, as
the former makes sure to give something meaningful for GADT
constructors. This makes the panic go away at the very least. There
is still an underlying issue with the way the internals of
`reifyDataCon` work, as described in
https://gitlab.haskell.org/ghc/ghc/issues/17305#note_227023, but we
leave that as future work, since fixing the underlying issue is
much trickier (see
https://gitlab.haskell.org/ghc/ghc/issues/17305#note_227087).

3 files changed, 20 insertions, 0 deletions

diff --git a/testsuite/tests/th/T17305.hs b/testsuite/tests/th/T17305.hs
new file mode 100644
index 0000000000..f7cc4cbcb0
--- /dev/null
+++ b/testsuite/tests/th/T17305.hs
@@ -0,0 +1,16 @@
+{-# LANGUAGE GADTs #-}
+{-# LANGUAGE TemplateHaskell #-}
+{-# LANGUAGE TypeFamilies #-}
+module T17305 where
+
+import Data.Kind
+import Language.Haskell.TH hiding (Type)
+import System.IO
+
+data family Foo a
+data instance Foo :: Type -> Type where
+  MkFoo :: Foo a
+
+$(do i <- reify ''Foo
+     runIO $ hPutStrLn stderr $ pprint i
+     pure [])
diff --git a/testsuite/tests/th/T17305.stderr b/testsuite/tests/th/T17305.stderr
new file mode 100644
index 0000000000..89618291ef
--- /dev/null
+++ b/testsuite/tests/th/T17305.stderr
@@ -0,0 +1,3 @@
+data family T17305.Foo (a_0 :: *) :: *
+data instance T17305.Foo where
+    T17305.MkFoo :: forall (a_1 :: *) . T17305.Foo a_1
diff --git a/testsuite/tests/th/all.T b/testsuite/tests/th/all.T
index 690270d732..127bdf665c 100644
--- a/testsuite/tests/th/all.T
+++ b/testsuite/tests/th/all.T
@@ -492,6 +492,7 @@ test('T16980a', normal, compile_fail, [''])
 test('T17270a', extra_files(['T17270.hs']), multimod_compile, ['T17270', '-v0'])
 test('T17270b', extra_files(['T17270.hs']), multimod_compile, ['T17270', '-fenable-th-splice-warnings -v0'])
 test('T17296', normal, compile, ['-v0'])
+test('T17305', normal, compile, ['-v0'])
 test('T17380', normal, compile_fail, [''])
 test('T17394', normal, compile, ['-v0 -ddump-splices -dsuppress-uniques'])
 test('T17379a', normal, compile_fail, [''])