Add -XImpredicativeTypes, and tighten up type-validity checking (cf Trac 2019)

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

diff --git a/compiler/typecheck/TcTyClsDecls.lhs b/compiler/typecheck/TcTyClsDecls.lhs
index 83b2114..89afedf 100644 (file)
--- a/compiler/typecheck/TcTyClsDecls.lhs
+++ b/compiler/typecheck/TcTyClsDecls.lhs
@@ -1054,6 +1054,9 @@ checkValidDataCon tc con
     addErrCtxt (dataConCtxt con)               $ 
     do { checkTc (dataConTyCon con == tc) (badDataConTyCon con)
        ; checkValidType ctxt (dataConUserType con)
+       ; checkValidMonoType (dataConOrigResTy con)
+               -- Disallow MkT :: T (forall a. a->a)
+               -- Reason: it's really the argument of an equality constraint
        ; ifM (isNewTyCon tc) (checkNewDataCon con)
     }
   where