Disallow linear arrows in GADT records (#19928)

Before this patch, GHC used to silently accept programs such as the
following:

	data R where
	  D1 :: { d1 :: Int } %1 -> R

The %1 annotation was completely ignored. Now it is a proper error.

One remaining issue is that in the error message (⊸) turns
into (%1 ->). This is to be corrected with upcoming exactprint updates.

6 files changed, 48 insertions, 11 deletions

diff --git a/compiler/GHC/Parser/Errors/Ppr.hs b/compiler/GHC/Parser/Errors/Ppr.hs
index 6a2152f3f7..6b08ae0877 100644
--- a/compiler/GHC/Parser/Errors/Ppr.hs
+++ b/compiler/GHC/Parser/Errors/Ppr.hs
@@ -520,6 +520,12 @@ instance Diagnostic PsMessage where
               text "character in package name"
             ]
 
+    PsErrIllegalGadtRecordMultiplicity arr
+      -> mkSimpleDecorated $ vcat
+            [ text "Parse error" <> colon <+> quotes (ppr arr)
+            , text "Record constructors in GADTs must use an ordinary, non-linear arrow."
+            ]
+
   diagnosticReason  = \case
     PsUnknownMessage m                            -> diagnosticReason m
     PsWarnTab{}                                   -> WarningWithFlag Opt_WarnTabs
@@ -628,6 +634,7 @@ instance Diagnostic PsMessage where
     PsErrUnexpectedTypeInDecl{}                   -> ErrorWithoutFlag
     PsErrInvalidPackageName{}                     -> ErrorWithoutFlag
     PsErrParseRightOpSectionInPat{}               -> ErrorWithoutFlag
+    PsErrIllegalGadtRecordMultiplicity{}          -> ErrorWithoutFlag
 
   diagnosticHints  = \case
     PsUnknownMessage m                            -> diagnosticHints m
@@ -754,6 +761,7 @@ instance Diagnostic PsMessage where
     PsErrInvalidTypeSignature{}                   -> noHints
     PsErrUnexpectedTypeInDecl{}                   -> noHints
     PsErrInvalidPackageName{}                     -> noHints
+    PsErrIllegalGadtRecordMultiplicity{}          -> noHints
 
 suggestParensAndBlockArgs :: [GhcHint]
 suggestParensAndBlockArgs =
diff --git a/compiler/GHC/Parser/Errors/Types.hs b/compiler/GHC/Parser/Errors/Types.hs
index d75c223253..38c54b7149 100644
--- a/compiler/GHC/Parser/Errors/Types.hs
+++ b/compiler/GHC/Parser/Errors/Types.hs
@@ -395,6 +395,9 @@ data PsMessage
    -- TODO: embed the proper operator, if possible
    | forall infixOcc. (OutputableBndr infixOcc) => PsErrParseRightOpSectionInPat !infixOcc !(PatBuilder GhcPs)
 
+   -- | Illegal linear arrow or multiplicity annotation in GADT record syntax
+   | PsErrIllegalGadtRecordMultiplicity !(HsArrow GhcPs)
+
 newtype StarIsType = StarIsType Bool
 
 -- | Extra details about a parse error, which helps
diff --git a/compiler/GHC/Parser/PostProcess.hs b/compiler/GHC/Parser/PostProcess.hs
index 34c973fefc..ffe44227fd 100644
--- a/compiler/GHC/Parser/PostProcess.hs
+++ b/compiler/GHC/Parser/PostProcess.hs
@@ -716,17 +716,22 @@ mkGadtDecl loc names ty annsIn = do
   cs <- getCommentsFor loc
   let l = noAnnSrcSpan loc
 
-  let (args, res_ty, annsa, csa)
-        | L ll (HsFunTy af _w (L loc' (HsRecTy an rf)) res_ty) <- body_ty
-        = let
-            an' = addTrailingAnnToL (locA loc') (anns af) (comments af) an
-          in ( RecConGADT (L (SrcSpanAnn an' (locA loc')) rf), res_ty
-             , [], epAnnComments (ann ll))
-        | otherwise
-        = let (anns, cs, arg_types, res_type) = splitHsFunType body_ty
-          in (PrefixConGADT arg_types, res_type, anns, cs)
-
-      an = case outer_bndrs of
+  (args, res_ty, annsa, csa) <-
+    case body_ty of
+     L ll (HsFunTy af hsArr (L loc' (HsRecTy an rf)) res_ty) -> do
+       let an' = addTrailingAnnToL (locA loc') (anns af) (comments af) an
+       case hsArr of
+         HsUnrestrictedArrow _ -> return ()
+         _ -> addError $ mkPlainErrorMsgEnvelope (getLocA body_ty) $
+                                 (PsErrIllegalGadtRecordMultiplicity hsArr)
+
+       return ( RecConGADT (L (SrcSpanAnn an' (locA loc')) rf), res_ty
+              , [], epAnnComments (ann ll))
+     _ -> do
+       let (anns, cs, arg_types, res_type) = splitHsFunType body_ty
+       return (PrefixConGADT arg_types, res_type, anns, cs)
+
+  let an = case outer_bndrs of
         _                -> EpAnn (spanAsAnchor loc) (annsIn ++ annsa) (cs Semi.<> csa)
 
   pure $ L l ConDeclGADT
diff --git a/testsuite/tests/parser/should_fail/T19928.hs b/testsuite/tests/parser/should_fail/T19928.hs
new file mode 100644
index 0000000000..c6c2947577
--- /dev/null
+++ b/testsuite/tests/parser/should_fail/T19928.hs
@@ -0,0 +1,8 @@
+{-# LANGUAGE UnicodeSyntax, LinearTypes #-}
+
+module T19928 where
+
+data R where
+  D1 :: { d1 :: Int } %1 -> R
+  Dp :: { dp :: Int } %p -> R
+  Dl :: { dl :: Int } ⊸ R
diff --git a/testsuite/tests/parser/should_fail/T19928.stderr b/testsuite/tests/parser/should_fail/T19928.stderr
new file mode 100644
index 0000000000..342639a100
--- /dev/null
+++ b/testsuite/tests/parser/should_fail/T19928.stderr
@@ -0,0 +1,12 @@
+
+T19928.hs:6:9: error:
+    Parse error: ‘(%1 ->)’
+    Record constructors in GADTs must use an ordinary, non-linear arrow.
+
+T19928.hs:7:9: error:
+    Parse error: ‘(%p ->)’
+    Record constructors in GADTs must use an ordinary, non-linear arrow.
+
+T19928.hs:8:9: error:
+    Parse error: ‘(%1 ->)’
+    Record constructors in GADTs must use an ordinary, non-linear arrow.
diff --git a/testsuite/tests/parser/should_fail/all.T b/testsuite/tests/parser/should_fail/all.T
index 49a298c93f..9975f6c5d7 100644
--- a/testsuite/tests/parser/should_fail/all.T
+++ b/testsuite/tests/parser/should_fail/all.T
@@ -190,3 +190,4 @@ test('RecordDotSyntaxFail11', normal, compile_fail, [''])
 test('RecordDotSyntaxFail12', normal, compile_fail, [''])
 test('RecordDotSyntaxFail13', normal, compile_fail, [''])
 test('T19504', normal, compile_fail, [''])
+test('T19928', normal, compile_fail, [''])