Fix Trac #3176: intercept naughty record selectors

[ghc-hetmet.git] / compiler / typecheck / TcPat.lhs

diff --git a/compiler/typecheck/TcPat.lhs b/compiler/typecheck/TcPat.lhs
index e21fb68..82ac5e3 100644 (file)
--- a/compiler/typecheck/TcPat.lhs
+++ b/compiler/typecheck/TcPat.lhs
@@ -37,6 +37,7 @@ import TyCon
 import DataCon
 import PrelNames
 import BasicTypes hiding (SuccessFlag(..))
+import DynFlags        ( DynFlag( Opt_GADTs ) )
 import SrcLoc
 import ErrUtils
 import Util
@@ -670,6 +671,12 @@ tcConPat pstate con_span data_con tycon pat_ty arg_pats thing_inside
              pstate' | no_equalities = pstate
                      | otherwise     = pstate { pat_eqs = True }
 
+        ; gadts_on <- doptM Opt_GADTs
+       ; checkTc (no_equalities || gadts_on)
+                 (ptext (sLit "A pattern match on a GADT requires -XGADTs"))
+                 -- Trac #2905 decided that a *pattern-match* of a GADT
+                 -- should require the GADT language flag
+
        ; unless no_equalities $ checkTc (isRigidTy pat_ty) $
                                  nonRigidMatch (pat_ctxt pstate) data_con