Remove prefix arrow support for GADTs (#17211)

This reverts the change in #9096.

The specialcasing done for prefix (->) is brittle and
does not support VTA, type families, type synonyms etc.

compiler/GHC/Hs/Types.hs

@@ -84,7 +84,6 @@ import Name( Name, NamedThing(getName) )
 import RdrName ( RdrName )
 import DataCon( HsSrcBang(..), HsImplBang(..),
                 SrcStrictness(..), SrcUnpackedness(..) )
-import TysPrim( funTyConName )
 import TysWiredIn( mkTupleStr )
 import Type
 import GHC.Hs.Doc
@@ -1151,8 +1150,6 @@ mkHsAppKindTy ext ty k
 -- splitHsFunType decomposes a type (t1 -> t2 ... -> tn)
 -- Breaks up any parens in the result type:
 --      splitHsFunType (a -> (b -> c)) = ([a,b], c)
--- Also deals with (->) t1 t2; that is why it only works on LHsType Name
---   (see #9096)
 splitHsFunType :: LHsType GhcRn -> ([LHsType GhcRn], LHsType GhcRn)
 splitHsFunType (L _ (HsParTy _ ty))
   = splitHsFunType ty
@@ -1160,19 +1157,6 @@ splitHsFunType (L _ (HsParTy _ ty))
 splitHsFunType (L _ (HsFunTy _ x y))
   | (args, res) <- splitHsFunType y
   = (x:args, res)
-{- This is not so correct, because it won't work with visible kind app, in case
-  someone wants to write '(->) @k1 @k2 t1 t2'. Fixing this would require changing
-  ConDeclGADT abstract syntax -}
-splitHsFunType orig_ty@(L _ (HsAppTy _ t1 t2))
-  = go t1 [t2]
-  where  -- Look for (->) t1 t2, possibly with parenthesisation
-    go (L _ (HsTyVar _ _ (L _ fn))) tys | fn == funTyConName
-                                 , [t1,t2] <- tys
-                                 , (args, res) <- splitHsFunType t2
-                                 = (t1:args, res)
-    go (L _ (HsAppTy _ t1 t2)) tys = go t1 (t2:tys)
-    go (L _ (HsParTy _ ty))    tys = go ty tys
-    go _                       _   = ([], orig_ty)  -- Failure to match
 
 splitHsFunType other = ([], other)
 

testsuite/tests/gadt/T9096.hs

-{-# LANGUAGE GADTs #-}
-
-module T9096 where
-
-data Foo a where
-  MkFoo :: (->) a (Foo a)

testsuite/tests/gadt/all.T

@@ -110,7 +110,6 @@ test('T7294', normal, compile, [''])
 test('T7321', [], makefile_test, [])
 test('T7974', normal, compile, [''])
 test('T7558', normal, compile_fail, [''])
-test('T9096', normal, compile, [''])
 test('T9380', normal, compile_and_run, [''])
 test('T12087', normal, compile_fail, [''])
 test('T12468', normal, compile_fail, [''])