Fix #14167 by using isGadtSyntaxTyCon in more places

Summary:
Two places in GHC effectively attempt to //guess// whether a data type
was declared using GADT syntax:

1. When reifying a data type in Template Haskell
2. When pretty-printing a data type (e.g., via `:info` in GHCi)

But there's no need for heuristics here, since we have a 100% accurate way to
determine whether a data type was declared using GADT syntax: the
`isGadtSyntaxTyCon` function! By simply using that as the metric, we obtain
far more accurate TH reification and pretty-printing results.

This is technically a breaking change, since Template Haskell reification will
now reify some data type constructors as `(Rec)GadtC` that it didn't before,
and some data type constructors that were previously reified as `(Rec)GadtC`
will no longer be reified as such. But it's a very understandable breaking
change, since the previous behavior was simply incorrect.

Test Plan: ./validate

Reviewers: simonpj, goldfire, austin, bgamari

Reviewed By: simonpj

Subscribers: rwbarton, thomie

GHC Trac Issues: #14167

Differential Revision: https://phabricator.haskell.org/D3901

7 files changed, 31 insertions, 34 deletions

diff --git a/testsuite/tests/ghci/scripts/T7730.stdout b/testsuite/tests/ghci/scripts/T7730.stdout
index e96e909413..bf9c1d025b 100644
--- a/testsuite/tests/ghci/scripts/T7730.stdout
+++ b/testsuite/tests/ghci/scripts/T7730.stdout
@@ -3,6 +3,5 @@ data A (x :: k) (y :: k1)
   	-- Defined at <interactive>:2:1
 A :: k1 -> k2 -> *
 type role T phantom
-data T (a :: k) where
-  MkT :: forall k (a :: k) a1. a1 -> T a
+data T (a :: k) = forall a1. MkT a1
   	-- Defined at <interactive>:6:1
diff --git a/testsuite/tests/ghci/scripts/T7873.stdout b/testsuite/tests/ghci/scripts/T7873.stdout
index 2c79056da4..bcdebe71e1 100644
--- a/testsuite/tests/ghci/scripts/T7873.stdout
+++ b/testsuite/tests/ghci/scripts/T7873.stdout
@@ -1,5 +1,5 @@
-data D2 where
-  MkD2 :: (forall (p :: k -> *) (a :: k). p a -> Int) -> D2
+data D2
+  = forall k. MkD2 (forall (p :: k -> *) (a :: k). p a -> Int)
   	-- Defined at <interactive>:3:1
 data D3 = MkD3 (forall k (p :: k -> *) (a :: k). p a -> Int)
   	-- Defined at <interactive>:4:1
diff --git a/testsuite/tests/ghci/scripts/T9181.stdout b/testsuite/tests/ghci/scripts/T9181.stdout
index d51b345d5c..1ae9bd54d0 100644
--- a/testsuite/tests/ghci/scripts/T9181.stdout
+++ b/testsuite/tests/ghci/scripts/T9181.stdout
@@ -4,19 +4,22 @@ type family GHC.TypeLits.AppendSymbol (a :: GHC.Types.Symbol)
 type family GHC.TypeLits.CmpSymbol (a :: GHC.Types.Symbol)
                                    (b :: GHC.Types.Symbol)
             :: Ordering
-data GHC.TypeLits.ErrorMessage where
-  GHC.TypeLits.Text :: GHC.Types.Symbol -> GHC.TypeLits.ErrorMessage
-  GHC.TypeLits.ShowType :: t -> GHC.TypeLits.ErrorMessage
-  (GHC.TypeLits.:<>:) :: GHC.TypeLits.ErrorMessage
-                         -> GHC.TypeLits.ErrorMessage -> GHC.TypeLits.ErrorMessage
-  (GHC.TypeLits.:$$:) :: GHC.TypeLits.ErrorMessage
-                         -> GHC.TypeLits.ErrorMessage -> GHC.TypeLits.ErrorMessage
+data GHC.TypeLits.ErrorMessage
+  = GHC.TypeLits.Text GHC.Types.Symbol
+  | forall t. GHC.TypeLits.ShowType t
+  | GHC.TypeLits.ErrorMessage
+    GHC.TypeLits.:<>:
+    GHC.TypeLits.ErrorMessage
+  | GHC.TypeLits.ErrorMessage
+    GHC.TypeLits.:$$:
+    GHC.TypeLits.ErrorMessage
 class GHC.TypeLits.KnownSymbol (n :: GHC.Types.Symbol) where
   GHC.TypeLits.symbolSing :: GHC.TypeLits.SSymbol n
   {-# MINIMAL symbolSing #-}
-data GHC.TypeLits.SomeSymbol where
-  GHC.TypeLits.SomeSymbol :: GHC.TypeLits.KnownSymbol n =>
-                             (Data.Proxy.Proxy n) -> GHC.TypeLits.SomeSymbol
+data GHC.TypeLits.SomeSymbol
+  = forall (n :: GHC.Types.Symbol).
+    GHC.TypeLits.KnownSymbol n =>
+    GHC.TypeLits.SomeSymbol (Data.Proxy.Proxy n)
 type family GHC.TypeLits.TypeError (a :: GHC.TypeLits.ErrorMessage)
             :: b
 GHC.TypeLits.natVal ::
@@ -54,9 +57,10 @@ class GHC.TypeNats.KnownNat (n :: GHC.Types.Nat) where
   GHC.TypeNats.natSing :: GHC.TypeNats.SNat n
   {-# MINIMAL natSing #-}
 data GHC.Types.Nat
-data GHC.TypeNats.SomeNat where
-  GHC.TypeNats.SomeNat :: GHC.TypeNats.KnownNat n =>
-                          (Data.Proxy.Proxy n) -> GHC.TypeNats.SomeNat
+data GHC.TypeNats.SomeNat
+  = forall (n :: GHC.Types.Nat).
+    GHC.TypeNats.KnownNat n =>
+    GHC.TypeNats.SomeNat (Data.Proxy.Proxy n)
 data GHC.Types.Symbol
 type family (GHC.TypeNats.^) (a :: GHC.Types.Nat)
                              (b :: GHC.Types.Nat)
diff --git a/testsuite/tests/ghci/scripts/ghci030.stdout b/testsuite/tests/ghci/scripts/ghci030.stdout
index 9344bc39bd..49ce606456 100644
--- a/testsuite/tests/ghci/scripts/ghci030.stdout
+++ b/testsuite/tests/ghci/scripts/ghci030.stdout
@@ -1,6 +1,2 @@
-data D where
-  C :: (Int -> a) -> Char -> D
-  	-- Defined at ghci030.hs:8:1
-data D where
-  C :: (Int -> a) -> Char -> D
-  	-- Defined at ghci030.hs:8:10
+data D = forall a. C (Int -> a) Char 	-- Defined at ghci030.hs:8:1
+data D = forall a. C (Int -> a) Char 	-- Defined at ghci030.hs:8:10
diff --git a/testsuite/tests/rename/should_fail/rnfail055.stderr b/testsuite/tests/rename/should_fail/rnfail055.stderr
index 7fc5d80bad..b9ba174519 100644
--- a/testsuite/tests/rename/should_fail/rnfail055.stderr
+++ b/testsuite/tests/rename/should_fail/rnfail055.stderr
@@ -71,10 +71,8 @@ RnFail055.hs-boot:25:1: error:
     Type constructor ‘T7’ has conflicting definitions in the module
     and its hs-boot file
     Main module: type role T7 phantom
-                 data T7 a where
-                   T7 :: a1 -> T7 a
-    Boot file:   data T7 a where
-                   T7 :: a -> T7 a
+                 data T7 a = forall a1. T7 a1
+    Boot file:   data T7 a = forall b. T7 a
     The roles do not match.
     Roles on abstract types default to ‘representational’ in boot files.
     The constructors do not match: The types for ‘T7’ differ
diff --git a/testsuite/tests/th/T4188.stderr b/testsuite/tests/th/T4188.stderr
index 2e4155fd8b..38a22cf172 100644
--- a/testsuite/tests/th/T4188.stderr
+++ b/testsuite/tests/th/T4188.stderr
@@ -1,8 +1,9 @@
-data T4188.T1 (a_0 :: *) = forall (b_1 :: *) . T4188.MkT1 a_0 b_1
-data T4188.T2 (a_0 :: *)
-    = forall (b_1 :: *) . (T4188.C a_0, T4188.C b_1) => T4188.MkT2 a_0
-                                                                   b_1
+data T4188.T1 (a_0 :: *) where
+    T4188.MkT1 :: forall (a_1 :: *) (b_2 :: *) . a_1 ->
+                                                 b_2 -> T4188.T1 a_1
+data T4188.T2 (a_0 :: *) where
+    T4188.MkT2 :: forall (a_1 :: *) (b_2 :: *) . (T4188.C a_1,
+                                                  T4188.C b_2) => a_1 -> b_2 -> T4188.T2 a_1
 data T4188.T3 (x_0 :: *) where
     T4188.MkT3 :: forall (x_1 :: *) (y_2 :: *) . (T4188.C x_1,
                                                   T4188.C y_2) => x_1 -> y_2 -> T4188.T3 (x_1, y_2)
-
diff --git a/testsuite/tests/typecheck/should_fail/T3468.stderr b/testsuite/tests/typecheck/should_fail/T3468.stderr
index 1f8bdcb11b..0a0fec223b 100644
--- a/testsuite/tests/typecheck/should_fail/T3468.stderr
+++ b/testsuite/tests/typecheck/should_fail/T3468.stderr
@@ -3,7 +3,6 @@ T3468.hs-boot:3:1: error:
     Type constructor ‘Tool’ has conflicting definitions in the module
     and its hs-boot file
     Main module: type role Tool phantom
-                 data Tool d where
-                   F :: a -> Tool d
+                 data Tool d = forall a r. F a
     Boot file:   data Tool
     The types have different kinds