Raise parse error for `data T where`.

Empty GADTs data declarations can't be identified in type checker. This
patch adds additional checks in parser and raise a parse error when
encounter empty GADTs declarations but extension `GADTs` is not enabled.

Only empty declarations are checked in parser to avoid affecting
existing
error messages related to missing GADTs extension.

This patch should fix issue 8258.

Signed-off-by: HE, Tao <sighingnow@gmail.com>

Test Plan: make test TEST="T8258 T8258NoGADTs"

Reviewers: bgamari, mpickering, alanz, RyanGlScott

Reviewed By: bgamari, RyanGlScott

Subscribers: adamse, RyanGlScott, rwbarton, thomie, mpickering, carter

GHC Trac Issues: #8258

Differential Revision: https://phabricator.haskell.org/D4350

1 files changed, 16 insertions, 0 deletions

diff --git a/compiler/parser/RdrHsSyn.hs b/compiler/parser/RdrHsSyn.hs
index 357d22438a..6ac6cbc974 100644
--- a/compiler/parser/RdrHsSyn.hs
+++ b/compiler/parser/RdrHsSyn.hs
@@ -55,6 +55,7 @@ module   RdrHsSyn (
         checkValSigLhs,
         checkDoAndIfThenElse,
         checkRecordSyntax,
+        checkEmptyGADTs,
         parseErrorSDoc, hintBangPat,
         splitTilde, splitTildeApps,
 
@@ -783,6 +784,21 @@ checkRecordSyntax lr@(L loc r)
                       (text "Illegal record syntax (use TraditionalRecordSyntax):" <+>
                        ppr r)
 
+-- | Check if the gadt_constrlist is empty. Only raise parse error for
+-- `data T where` to avoid affecting existing error message, see #8258.
+checkEmptyGADTs :: Located ([AddAnn], [LConDecl GhcPs])
+                -> P (Located ([AddAnn], [LConDecl GhcPs]))
+checkEmptyGADTs gadts@(L span (_, []))               -- Empty GADT declaration.
+    = do opts <- fmap options getPState
+         if LangExt.GADTSyntax `extopt` opts         -- GADTs implies GADTSyntax
+            then return gadts
+            else parseErrorSDoc span $ vcat
+              [ text "Illegal keyword 'where' in data declaration"
+              , text "Perhaps you intended to use GADTs or a similar language"
+              , text "extension to enable syntax: data T where"
+              ]
+checkEmptyGADTs gadts = return gadts              -- Ordinary GADT declaration.
+
 checkTyClHdr :: Bool               -- True  <=> class header
                                    -- False <=> type header
              -> LHsType GhcPs