Check that type variable does not reference itself in its kind signature

This fixes #11592.

Test Plan: validate

Reviewers: simonpj, austin, bgamari, goldfire

Reviewed By: goldfire

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2914

GHC Trac Issues: #11592

1 files changed, 16 insertions, 6 deletions

diff --git a/compiler/rename/RnTypes.hs b/compiler/rename/RnTypes.hs
index f3fcf88ade..ec00511d62 100644
--- a/compiler/rename/RnTypes.hs
+++ b/compiler/rename/RnTypes.hs
@@ -918,19 +918,19 @@ bindLHsTyVarBndr :: HsDocContext
 bindLHsTyVarBndr doc mb_assoc kv_names tv_names hs_tv_bndr thing_inside
   = case hs_tv_bndr of
       L loc (UserTyVar lrdr@(L lv rdr)) ->
-        do { check_dup loc rdr
+        do { check_dup loc rdr []
            ; nm <- newTyVarNameRn mb_assoc lrdr
            ; bindLocalNamesFV [nm] $
              thing_inside [] emptyNameSet (L loc (UserTyVar (L lv nm))) }
       L loc (KindedTyVar lrdr@(L lv rdr) kind) ->
-        do { check_dup lv rdr
+        do { free_kvs <- freeKiTyVarsAllVars <$> extractHsTyRdrTyVars kind
+           ; check_dup lv rdr (map unLoc free_kvs)
 
              -- check for -XKindSignatures
            ; sig_ok <- xoptM LangExt.KindSignatures
            ; unless sig_ok (badKindSigErr doc kind)
 
              -- deal with kind vars in the user-written kind
-           ; free_kvs <- freeKiTyVarsAllVars <$> extractHsTyRdrTyVars kind
            ; bindImplicitKvs doc mb_assoc free_kvs tv_names $
              \ new_kv_nms other_kv_nms ->
              do { (kind', fvs1) <- rnLHsKind doc kind
@@ -943,9 +943,15 @@ bindLHsTyVarBndr doc mb_assoc kv_names tv_names hs_tv_bndr thing_inside
       -- make sure that the RdrName isn't in the sets of
       -- names. We can't just check that it's not in scope at all
       -- because we might be inside an associated class.
-    check_dup :: SrcSpan -> RdrName -> RnM ()
-    check_dup loc rdr
-      = do { m_name <- lookupLocalOccRn_maybe rdr
+    check_dup :: SrcSpan -> RdrName -> [RdrName] -> RnM ()
+    check_dup loc rdr kindFreeVars
+      = do { -- Disallow use of a type variable name in its
+             -- kind signature (#11592).
+             when (rdr `elem` kindFreeVars) $
+             addErrAt loc (vcat [ ki_ty_self_err rdr
+                                , pprHsDocContext doc ])
+
+           ; m_name <- lookupLocalOccRn_maybe rdr
            ; whenIsJust m_name $ \name ->
         do { when (name `elemNameSet` kv_names) $
              addErrAt loc (vcat [ ki_ty_err_msg name
@@ -957,6 +963,10 @@ bindLHsTyVarBndr doc mb_assoc kv_names tv_names hs_tv_bndr thing_inside
                       text "used as a kind variable before being bound" $$
                       text "as a type variable. Perhaps reorder your variables?"
 
+    ki_ty_self_err n = text "Variable" <+> quotes (ppr n) <+>
+                       text "is used in the kind signature of its" $$
+                       text "declaration as a type variable."
+
 
 bindImplicitKvs :: HsDocContext
                 -> Maybe a