From 9fd87ef8a16fbbce35205ae63d75d239bb575ccc Mon Sep 17 00:00:00 2001
From: Ryan Scott <ryan.gl.scott@gmail.com>
Date: Tue, 24 Jan 2017 10:16:38 -0500
Subject: Don't put foralls in front of TH-spliced GADT constructors that don't
 need them

Summary:
It turns out that D2974 broke this program
(see https://phabricator.haskell.org/rGHC729a5e452db5#58801):

```lang=haskell
{-# LANGUAGE ConstraintKinds #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE TemplateHaskell #-}
{-# OPTIONS_GHC -ddump-splices #-}
module Bug where

import GHC.Exts (Constraint)

$([d| data Dec13 :: (* -> Constraint) -> * where
        MkDec13 :: c a => a -> Dec13 c
    |])
```

This was actually due to a long-standing bug in `hsSyn/Convert` that put
unnecessary `forall`s in front of GADT constructors that didn't have any
explicitly quantified type variables.

This cargo-cults the code in `Convert` that handles `ForallT` and adapts
it to `ForallC`. Fixes #13123 (for real this time).

Test Plan: make test TEST=T13123

Reviewers: goldfire, austin, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D3002

GHC Trac Issues: #13123
---
 compiler/hsSyn/Convert.hs    | 15 +++++++++++----
 testsuite/tests/th/T13123.hs |  7 +++++++
 2 files changed, 18 insertions(+), 4 deletions(-)

diff --git a/compiler/hsSyn/Convert.hs b/compiler/hsSyn/Convert.hs
index 7749265682..3e0bf126cf 100644
--- a/compiler/hsSyn/Convert.hs
+++ b/compiler/hsSyn/Convert.hs
@@ -510,10 +510,17 @@ cvtConstr (ForallC tvs ctxt con)
         ; L _ con'    <- cvtConstr con
         ; returnL $ case con' of
                 ConDeclGADT { con_type = conT } ->
-                  con' { con_type =
-                         HsIB PlaceHolder
-                         (noLoc $ HsForAllTy (hsq_explicit tvs') $
-                          (noLoc $ HsQualTy (L loc ctxt') (hsib_body conT))) }
+                  let hs_ty
+                        | null tvs = rho_ty
+                        | otherwise = noLoc $ HsForAllTy
+                                                { hst_bndrs = hsq_explicit tvs'
+                                                , hst_body  = rho_ty }
+                      rho_ty
+                        | null ctxt = hsib_body conT
+                        | otherwise = noLoc $ HsQualTy
+                                                { hst_ctxt = L loc ctxt'
+                                                , hst_body = hsib_body conT }
+                  in con' { con_type = HsIB PlaceHolder hs_ty }
                 ConDeclH98  {} ->
                   let qvars = case (tvs, con_qvars con') of
                         ([], Nothing) -> Nothing
diff --git a/testsuite/tests/th/T13123.hs b/testsuite/tests/th/T13123.hs
index 987283be70..d7e1006b9e 100644
--- a/testsuite/tests/th/T13123.hs
+++ b/testsuite/tests/th/T13123.hs
@@ -1,3 +1,4 @@
+{-# LANGUAGE ConstraintKinds #-}
 {-# LANGUAGE DefaultSignatures #-}
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE PolyKinds #-}
@@ -5,6 +6,8 @@
 {-# LANGUAGE TemplateHaskell #-}
 module T13123 where
 
+import GHC.Exts (Constraint)
+
 $([d| idProxy :: forall proxy (a :: k). proxy a -> proxy a
       idProxy x = x
     |])
@@ -28,3 +31,7 @@ $([d| class Foo b where
 $([d| data GADT where
         MkGADT :: forall proxy (a :: k). proxy a -> GADT
     |])
+
+$([d| data Dec13 :: (* -> Constraint) -> * where
+        MkDec13 :: c a => a -> Dec13 c
+    |])
-- 
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