Make GADT constructors adhere to the forall-or-nothing rule properly

Issue #18191 revealed that the types of GADT constructors don't quite
adhere to the `forall`-or-nothing rule. This patch serves to clean up
this sad state of affairs somewhat. The main change is not in the
code itself, but in the documentation, as this patch introduces two
sections to the GHC User's Guide:

* A "Formal syntax for GADTs" section that presents a BNF-style
  grammar for what is and isn't allowed in GADT constructor types.
  This mostly exists to codify GHC's existing behavior, but it also
  imposes a new restriction that addresses #18191: the outermost
  `forall` and/or context in a GADT constructor is not allowed to be
  surrounded by parentheses. Doing so would make these
  `forall`s/contexts nested, and GADTs do not support nested
  `forall`s/contexts at present.

* A "`forall`-or-nothing rule" section that describes exactly what
  the `forall`-or-nothing rule is all about. Surprisingly, there was
  no mention of this anywhere in the User's Guide up until now!

To adhere the new specification in the "Formal syntax for GADTs"
section of the User's Guide, the following code changes were made:

* A new function, `GHC.Hs.Type.splitLHsGADTPrefixTy`, was introduced.
  This is very much like `splitLHsSigmaTy`, except that it avoids
  splitting apart any parentheses, which can be syntactically
  significant for GADT types. See
  `Note [No nested foralls or contexts in GADT constructors]` in
  `GHC.Hs.Type`.

* `ConDeclGADTPrefixPs`, an extension constructor for `XConDecl`, was
  introduced so that `GHC.Parser.PostProcess.mkGadtDecl` can return
  it when given a prefix GADT constructor. Unlike `ConDeclGADT`,
  `ConDeclGADTPrefixPs` does not split the GADT type into its argument
  and result types, as this cannot be done until after the type is
  renamed (see `Note [GADT abstract syntax]` in `GHC.Hs.Decls` for why
  this is the case).

* `GHC.Renamer.Module.rnConDecl` now has an additional case for
  `ConDeclGADTPrefixPs` that (1) splits apart the full `LHsType` into
  its `forall`s, context, argument types, and result type, and
  (2) checks for nested `forall`s/contexts. Step (2) used to be
  performed the typechecker (in `GHC.Tc.TyCl.badDataConTyCon`) rather
  than the renamer, but now the relevant code from the typechecker
  can simply be deleted.

  One nice side effect of this change is that we are able to give a
  more accurate error message for GADT constructors that use visible
  dependent quantification (e.g., `MkFoo :: forall a -> a -> Foo a`),
  which improves the stderr in the `T16326_Fail6` test case.

Fixes #18191. Bumps the Haddock submodule.

12 files changed, 156 insertions, 139 deletions

diff --git a/testsuite/tests/dependent/should_fail/T16326_Fail6.stderr b/testsuite/tests/dependent/should_fail/T16326_Fail6.stderr
index bb78e2f457..e4acd7a7fd 100644
--- a/testsuite/tests/dependent/should_fail/T16326_Fail6.stderr
+++ b/testsuite/tests/dependent/should_fail/T16326_Fail6.stderr
@@ -1,7 +1,5 @@
 
-T16326_Fail6.hs:9:3: error:
-    • GADT constructor type signature cannot contain nested ‘forall’s or contexts
-      Suggestion: instead use this type signature:
-        MkFoo :: forall a. a -> Foo a
-    • In the definition of data constructor ‘MkFoo’
-      In the data type declaration for ‘Foo’
+T16326_Fail6.hs:9:12: error:
+    Illegal visible, dependent quantification in the type of a term
+    (GHC does not yet support this)
+    In the definition of data constructor ‘MkFoo’
diff --git a/testsuite/tests/gadt/T12087.stderr b/testsuite/tests/gadt/T12087.stderr
index 0039e98657..ef0251a003 100644
--- a/testsuite/tests/gadt/T12087.stderr
+++ b/testsuite/tests/gadt/T12087.stderr
@@ -1,35 +1,20 @@
 
-T12087.hs:6:3: error:
-    • GADT constructor type signature cannot contain nested ‘forall’s or contexts
-      Suggestion: instead use this type signature:
-        MkF1 :: forall a. (Ord a, Eq a) => a -> F1 a
-    • In the definition of data constructor ‘MkF1’
-      In the data type declaration for ‘F1’
+T12087.hs:6:20: error:
+    GADT constructor type signature cannot contain nested ‘forall’s or contexts
+    In the definition of data constructor ‘MkF1’
 
-T12087.hs:9:3: error:
-    • GADT constructor type signature cannot contain nested ‘forall’s or contexts
-      Suggestion: instead use this type signature:
-        MkF2 :: forall a. (Ord a, Eq a) => a -> F2 a
-    • In the definition of data constructor ‘MkF2’
-      In the data type declaration for ‘F2’
+T12087.hs:9:25: error:
+    GADT constructor type signature cannot contain nested ‘forall’s or contexts
+    In the definition of data constructor ‘MkF2’
 
-T12087.hs:12:3: error:
-    • GADT constructor type signature cannot contain nested ‘forall’s or contexts
-      Suggestion: instead use this type signature:
-        MkF3 :: forall a b. (Eq a, Eq b) => a -> b -> F3 a
-    • In the definition of data constructor ‘MkF3’
-      In the data type declaration for ‘F3’
+T12087.hs:12:34: error:
+    GADT constructor type signature cannot contain nested ‘forall’s or contexts
+    In the definition of data constructor ‘MkF3’
 
-T12087.hs:15:3: error:
-    • GADT constructor type signature cannot contain nested ‘forall’s or contexts
-      Suggestion: instead use this type signature:
-        MkF4 :: forall a b. (Eq a, Eq b) => a -> b -> F4 a
-    • In the definition of data constructor ‘MkF4’
-      In the data type declaration for ‘F4’
+T12087.hs:15:36: error:
+    GADT constructor type signature cannot contain nested ‘forall’s or contexts
+    In the definition of data constructor ‘MkF4’
 
-T12087.hs:18:3: error:
-    • GADT constructor type signature cannot contain nested ‘forall’s or contexts
-      Suggestion: instead use this type signature:
-        MkF5 :: forall a b. Int -> Int -> a -> Int -> Int -> b -> F5 a
-    • In the definition of data constructor ‘MkF5’
-      In the data type declaration for ‘F5’
+T12087.hs:18:25: error:
+    GADT constructor type signature cannot contain nested ‘forall’s or contexts
+    In the definition of data constructor ‘MkF5’
diff --git a/testsuite/tests/gadt/T14320.hs b/testsuite/tests/gadt/T14320.hs
index 77c68ee92e..270c92c37c 100644
--- a/testsuite/tests/gadt/T14320.hs
+++ b/testsuite/tests/gadt/T14320.hs
@@ -10,8 +10,8 @@ data Exp :: Type where
 newtype TypedExp :: Type -> Type where
   TEGood ::  forall a . (Exp -> (TypedExp a))
 
--- The only difference here is that the type is wrapped in parentheses,
--- but GHC 8.0.1 rejects this program
+-- The presence of outer parentheses makes the `forall` nested, and
+-- GADTs do not permit nested `forall`s.
 --
 newtype TypedExpToo :: Type -> Type where
   TEBad  :: (forall a . (Exp -> (TypedExpToo a)))
diff --git a/testsuite/tests/gadt/T14320.stderr b/testsuite/tests/gadt/T14320.stderr
new file mode 100644
index 0000000000..9cfb6ed9fc
--- /dev/null
+++ b/testsuite/tests/gadt/T14320.stderr
@@ -0,0 +1,4 @@
+
+T14320.hs:17:14: error:
+    GADT constructor type signature cannot contain nested ‘forall’s or contexts
+    In the definition of data constructor ‘TEBad’
diff --git a/testsuite/tests/gadt/T16427.stderr b/testsuite/tests/gadt/T16427.stderr
index 1c80190e29..b7288c9cd6 100644
--- a/testsuite/tests/gadt/T16427.stderr
+++ b/testsuite/tests/gadt/T16427.stderr
@@ -1,7 +1,4 @@
 
-T16427.hs:5:14: error:
-    • GADT constructor type signature cannot contain nested ‘forall’s or contexts
-      Suggestion: instead use this type signature:
-        C :: forall b. Int -> b -> D
-    • In the definition of data constructor ‘C’
-      In the data type declaration for ‘D’
+T16427.hs:5:26: error:
+    GADT constructor type signature cannot contain nested ‘forall’s or contexts
+    In the definition of data constructor ‘C’
diff --git a/testsuite/tests/gadt/T18191.hs b/testsuite/tests/gadt/T18191.hs
new file mode 100644
index 0000000000..e30c7ad5b1
--- /dev/null
+++ b/testsuite/tests/gadt/T18191.hs
@@ -0,0 +1,16 @@
+{-# LANGUAGE GADTs #-}
+{-# LANGUAGE RankNTypes #-}
+module T18191 where
+
+data T where
+  MkT :: (forall a. a -> b -> T)
+
+data S a where
+  MkS :: (forall a. S a)
+
+data U a where
+  MkU :: (Show a => U a)
+
+data Z a where
+  MkZ1 :: forall a. forall b. { unZ1 :: (a, b) } -> Z (a, b)
+  MkZ2 :: Eq a => Eq b => { unZ1 :: (a, b) } -> Z (a, b)
diff --git a/testsuite/tests/gadt/T18191.stderr b/testsuite/tests/gadt/T18191.stderr
new file mode 100644
index 0000000000..b8c6c60bdc
--- /dev/null
+++ b/testsuite/tests/gadt/T18191.stderr
@@ -0,0 +1,20 @@
+
+T18191.hs:6:11: error:
+    GADT constructor type signature cannot contain nested ‘forall’s or contexts
+    In the definition of data constructor ‘MkT’
+
+T18191.hs:9:11: error:
+    GADT constructor type signature cannot contain nested ‘forall’s or contexts
+    In the definition of data constructor ‘MkS’
+
+T18191.hs:12:11: error:
+    GADT constructor type signature cannot contain nested ‘forall’s or contexts
+    In the definition of data constructor ‘MkU’
+
+T18191.hs:15:21: error:
+    GADT constructor type signature cannot contain nested ‘forall’s or contexts
+    In the definition of data constructor ‘MkZ1’
+
+T18191.hs:16:19: error:
+    GADT constructor type signature cannot contain nested ‘forall’s or contexts
+    In the definition of data constructor ‘MkZ2’
diff --git a/testsuite/tests/gadt/all.T b/testsuite/tests/gadt/all.T
index 29bde94100..3c146820ae 100644
--- a/testsuite/tests/gadt/all.T
+++ b/testsuite/tests/gadt/all.T
@@ -113,10 +113,11 @@ test('T7558', normal, compile_fail, [''])
 test('T9380', normal, compile_and_run, [''])
 test('T12087', normal, compile_fail, [''])
 test('T12468', normal, compile_fail, [''])
-test('T14320', normal, compile, [''])
+test('T14320', normal, compile_fail, [''])
 test('T14719', normal, compile_fail, ['-fdiagnostics-show-caret'])
 test('T14808', normal, compile, [''])
 test('T15009', normal, compile, [''])
 test('T15558', normal, compile, [''])
 test('T16427', normal, compile_fail, [''])
 test('T17423', expect_broken(17423), compile_and_run, [''])
+test('T18191', normal, compile_fail, [''])
diff --git a/testsuite/tests/ghc-api/annotations/T10399.stdout b/testsuite/tests/ghc-api/annotations/T10399.stdout
index a71abc4139..7588393264 100644
--- a/testsuite/tests/ghc-api/annotations/T10399.stdout
+++ b/testsuite/tests/ghc-api/annotations/T10399.stdout
@@ -34,9 +34,9 @@
 ((Test10399.hs:12:30,AnnComma), [Test10399.hs:12:30]),
 ((Test10399.hs:12:31-32,AnnCloseP), [Test10399.hs:12:32]),
 ((Test10399.hs:12:31-32,AnnOpenP), [Test10399.hs:12:31]),
-((Test10399.hs:(14,1)-(18,55),AnnData), [Test10399.hs:14:1-4]),
-((Test10399.hs:(14,1)-(18,55),AnnSemi), [Test10399.hs:20:1]),
-((Test10399.hs:(14,1)-(18,55),AnnWhere), [Test10399.hs:14:21-25]),
+((Test10399.hs:(14,1)-(18,53),AnnData), [Test10399.hs:14:1-4]),
+((Test10399.hs:(14,1)-(18,53),AnnSemi), [Test10399.hs:20:1]),
+((Test10399.hs:(14,1)-(18,53),AnnWhere), [Test10399.hs:14:21-25]),
 ((Test10399.hs:15:5-64,AnnDcolon), [Test10399.hs:15:11-12]),
 ((Test10399.hs:15:5-64,AnnSemi), [Test10399.hs:16:5]),
 ((Test10399.hs:15:14-64,AnnDot), [Test10399.hs:15:23]),
@@ -48,37 +48,29 @@
 ((Test10399.hs:15:45-46,AnnBang), [Test10399.hs:15:45]),
 ((Test10399.hs:15:45-46,AnnRarrow), [Test10399.hs:15:48-49]),
 ((Test10399.hs:15:45-64,AnnRarrow), [Test10399.hs:15:48-49]),
-((Test10399.hs:(16,5)-(17,69),AnnCloseP), [Test10399.hs:17:69]),
-((Test10399.hs:(16,5)-(17,69),AnnDcolon), [Test10399.hs:16:12-13]),
-((Test10399.hs:(16,5)-(17,69),AnnOpenP), [Test10399.hs:16:27]),
-((Test10399.hs:(16,5)-(17,69),AnnSemi), [Test10399.hs:18:5]),
-((Test10399.hs:(16,15)-(17,69),AnnDot), [Test10399.hs:16:25]),
-((Test10399.hs:(16,15)-(17,69),AnnForall), [Test10399.hs:16:15-20]),
-((Test10399.hs:(16,27)-(17,69),AnnCloseP), [Test10399.hs:17:69]),
-((Test10399.hs:(16,27)-(17,69),AnnOpenP), [Test10399.hs:16:27]),
-((Test10399.hs:16:28-43,AnnCloseP), [Test10399.hs:16:43, Test10399.hs:16:43]),
-((Test10399.hs:16:28-43,AnnDarrow), [Test10399.hs:16:45-46]),
-((Test10399.hs:16:28-43,AnnOpenP), [Test10399.hs:16:28, Test10399.hs:16:28]),
-((Test10399.hs:16:30-33,AnnComma), [Test10399.hs:16:34]),
-((Test10399.hs:16:48,AnnRarrow), [Test10399.hs:16:50-51]),
-((Test10399.hs:(16,48)-(17,68),AnnRarrow), [Test10399.hs:16:50-51]),
-((Test10399.hs:16:53-66,AnnRarrow), [Test10399.hs:17:45-46]),
-((Test10399.hs:(16,53)-(17,68),AnnRarrow), [Test10399.hs:17:45-46]),
-((Test10399.hs:17:48,AnnRarrow), [Test10399.hs:17:50-51]),
-((Test10399.hs:17:48-68,AnnRarrow), [Test10399.hs:17:50-51]),
-((Test10399.hs:17:66-68,AnnCloseS), [Test10399.hs:17:68]),
-((Test10399.hs:17:66-68,AnnOpenS), [Test10399.hs:17:66]),
-((Test10399.hs:18:5-55,AnnCloseP), [Test10399.hs:18:55]),
-((Test10399.hs:18:5-55,AnnDcolon), [Test10399.hs:18:16-17]),
-((Test10399.hs:18:5-55,AnnOpenP), [Test10399.hs:18:19]),
-((Test10399.hs:18:19-55,AnnCloseP), [Test10399.hs:18:55]),
-((Test10399.hs:18:19-55,AnnOpenP), [Test10399.hs:18:19]),
-((Test10399.hs:18:20-54,AnnDot), [Test10399.hs:18:29]),
-((Test10399.hs:18:20-54,AnnForall), [Test10399.hs:18:20-25]),
-((Test10399.hs:18:31-36,AnnCloseP), [Test10399.hs:18:36]),
-((Test10399.hs:18:31-36,AnnOpenP), [Test10399.hs:18:31]),
-((Test10399.hs:18:31-36,AnnRarrow), [Test10399.hs:18:38-39]),
-((Test10399.hs:18:31-54,AnnRarrow), [Test10399.hs:18:38-39]),
+((Test10399.hs:(16,5)-(17,67),AnnDcolon), [Test10399.hs:16:12-13]),
+((Test10399.hs:(16,5)-(17,67),AnnSemi), [Test10399.hs:18:5]),
+((Test10399.hs:(16,15)-(17,67),AnnDot), [Test10399.hs:16:25]),
+((Test10399.hs:(16,15)-(17,67),AnnForall), [Test10399.hs:16:15-20]),
+((Test10399.hs:16:27-42,AnnCloseP), [Test10399.hs:16:42, Test10399.hs:16:42]),
+((Test10399.hs:16:27-42,AnnDarrow), [Test10399.hs:16:44-45]),
+((Test10399.hs:16:27-42,AnnOpenP), [Test10399.hs:16:27, Test10399.hs:16:27]),
+((Test10399.hs:16:29-32,AnnComma), [Test10399.hs:16:33]),
+((Test10399.hs:16:47,AnnRarrow), [Test10399.hs:16:49-50]),
+((Test10399.hs:(16,47)-(17,67),AnnRarrow), [Test10399.hs:16:49-50]),
+((Test10399.hs:16:52-65,AnnRarrow), [Test10399.hs:17:44-45]),
+((Test10399.hs:(16,52)-(17,67),AnnRarrow), [Test10399.hs:17:44-45]),
+((Test10399.hs:17:47,AnnRarrow), [Test10399.hs:17:49-50]),
+((Test10399.hs:17:47-67,AnnRarrow), [Test10399.hs:17:49-50]),
+((Test10399.hs:17:65-67,AnnCloseS), [Test10399.hs:17:67]),
+((Test10399.hs:17:65-67,AnnOpenS), [Test10399.hs:17:65]),
+((Test10399.hs:18:5-53,AnnDcolon), [Test10399.hs:18:16-17]),
+((Test10399.hs:18:19-53,AnnDot), [Test10399.hs:18:28]),
+((Test10399.hs:18:19-53,AnnForall), [Test10399.hs:18:19-24]),
+((Test10399.hs:18:30-35,AnnCloseP), [Test10399.hs:18:35]),
+((Test10399.hs:18:30-35,AnnOpenP), [Test10399.hs:18:30]),
+((Test10399.hs:18:30-35,AnnRarrow), [Test10399.hs:18:37-38]),
+((Test10399.hs:18:30-53,AnnRarrow), [Test10399.hs:18:37-38]),
 ((Test10399.hs:20:1-25,AnnCloseQ), [Test10399.hs:20:24-25]),
 ((Test10399.hs:20:1-25,AnnOpen), [Test10399.hs:20:1-3]),
 ((Test10399.hs:20:1-25,AnnSemi), [Test10399.hs:22:1]),
diff --git a/testsuite/tests/ghc-api/annotations/Test10399.hs b/testsuite/tests/ghc-api/annotations/Test10399.hs
index 6a35712bfd..bb3265000d 100644
--- a/testsuite/tests/ghc-api/annotations/Test10399.hs
+++ b/testsuite/tests/ghc-api/annotations/Test10399.hs
@@ -13,9 +13,9 @@ mkPoli = mkBila . map ((,,(),,()) <$> P.base <*> P.pos <*> P.form)
 
 data MaybeDefault v where
     SetTo :: forall v . ( Eq v, Show v ) => !v -> MaybeDefault v
-    SetTo4 :: forall v a. (( Eq v, Show v ) => v -> MaybeDefault v
-                                            -> a -> MaybeDefault [a])
-    TestParens :: (forall v . (Eq v) -> MaybeDefault v)
+    SetTo4 :: forall v a. ( Eq v, Show v ) => v -> MaybeDefault v
+                                           -> a -> MaybeDefault [a]
+    TestParens :: forall v . (Eq v) -> MaybeDefault v
 
 [t| Map.Map T.Text $tc |]
 
diff --git a/testsuite/tests/parser/should_compile/T15323.hs b/testsuite/tests/parser/should_compile/T15323.hs
index ffc8ad85f0..42eb616141 100644
--- a/testsuite/tests/parser/should_compile/T15323.hs
+++ b/testsuite/tests/parser/should_compile/T15323.hs
@@ -3,4 +3,4 @@
 module T15323 where
 
 data MaybeDefault v where
-    TestParens  :: (forall v . (Eq v) => MaybeDefault v)
+    TestParens  :: forall v . (Eq v) => MaybeDefault v
diff --git a/testsuite/tests/parser/should_compile/T15323.stderr b/testsuite/tests/parser/should_compile/T15323.stderr
index c69f94afba..c07f2d6a43 100644
--- a/testsuite/tests/parser/should_compile/T15323.stderr
+++ b/testsuite/tests/parser/should_compile/T15323.stderr
@@ -8,7 +8,7 @@
     {ModuleName: T15323}))
   (Nothing)
   []
-  [({ T15323.hs:(5,1)-(6,56) }
+  [({ T15323.hs:(5,1)-(6,54) }
     (TyClD
      (NoExtField)
      (DataDecl
@@ -33,63 +33,67 @@
         [])
        (Nothing)
        (Nothing)
-       [({ T15323.hs:6:5-56 }
-         (ConDeclGADT
-          (NoExtField)
-          [({ T15323.hs:6:5-14 }
-            (Unqual
-             {OccName: TestParens}))]
-          ({ T15323.hs:6:21-55 }
-           (True))
-          [({ T15323.hs:6:28 }
-            (UserTyVar
-             (NoExtField)
-             (SpecifiedSpec)
-             ({ T15323.hs:6:28 }
-              (Unqual
-               {OccName: v}))))]
-          (Just
-           ({ T15323.hs:6:32-37 }
-            [({ T15323.hs:6:32-37 }
-              (HsParTy
-               (NoExtField)
-               ({ T15323.hs:6:33-36 }
-                (HsAppTy
-                 (NoExtField)
-                 ({ T15323.hs:6:33-34 }
-                  (HsTyVar
-                   (NoExtField)
-                   (NotPromoted)
-                   ({ T15323.hs:6:33-34 }
-                    (Unqual
-                     {OccName: Eq}))))
-                 ({ T15323.hs:6:36 }
-                  (HsTyVar
-                   (NoExtField)
-                   (NotPromoted)
-                   ({ T15323.hs:6:36 }
-                    (Unqual
-                     {OccName: v}))))))))]))
-          (PrefixCon
-           [])
-          ({ T15323.hs:6:42-55 }
-           (HsAppTy
+       [({ T15323.hs:6:5-54 }
+         (XConDecl
+          (ConDeclGADTPrefixPs
+           [({ T15323.hs:6:5-14 }
+             (Unqual
+              {OccName: TestParens}))]
+           (HsIB
             (NoExtField)
-            ({ T15323.hs:6:42-53 }
-             (HsTyVar
+            ({ T15323.hs:6:20-54 }
+             (HsForAllTy
               (NoExtField)
-              (NotPromoted)
-              ({ T15323.hs:6:42-53 }
-               (Unqual
-                {OccName: MaybeDefault}))))
-            ({ T15323.hs:6:55 }
-             (HsTyVar
-              (NoExtField)
-              (NotPromoted)
-              ({ T15323.hs:6:55 }
-               (Unqual
-                {OccName: v}))))))
-          (Nothing)))]
+              (ForallInvis)
+              [({ T15323.hs:6:27 }
+                (UserTyVar
+                 (NoExtField)
+                 (SpecifiedSpec)
+                 ({ T15323.hs:6:27 }
+                  (Unqual
+                   {OccName: v}))))]
+              ({ T15323.hs:6:31-54 }
+               (HsQualTy
+                (NoExtField)
+                ({ T15323.hs:6:31-36 }
+                 [({ T15323.hs:6:31-36 }
+                   (HsParTy
+                    (NoExtField)
+                    ({ T15323.hs:6:32-35 }
+                     (HsAppTy
+                      (NoExtField)
+                      ({ T15323.hs:6:32-33 }
+                       (HsTyVar
+                        (NoExtField)
+                        (NotPromoted)
+                        ({ T15323.hs:6:32-33 }
+                         (Unqual
+                          {OccName: Eq}))))
+                      ({ T15323.hs:6:35 }
+                       (HsTyVar
+                        (NoExtField)
+                        (NotPromoted)
+                        ({ T15323.hs:6:35 }
+                         (Unqual
+                          {OccName: v}))))))))])
+                ({ T15323.hs:6:41-54 }
+                 (HsAppTy
+                  (NoExtField)
+                  ({ T15323.hs:6:41-52 }
+                   (HsTyVar
+                    (NoExtField)
+                    (NotPromoted)
+                    ({ T15323.hs:6:41-52 }
+                     (Unqual
+                      {OccName: MaybeDefault}))))
+                  ({ T15323.hs:6:54 }
+                   (HsTyVar
+                    (NoExtField)
+                    (NotPromoted)
+                    ({ T15323.hs:6:54 }
+                     (Unqual
+                      {OccName: v})))))))))))
+           (Nothing))))]
        ({ <no location info> }
         [])))))]
   (Nothing)