[project @ 2002-06-21 13:31:50 by simonpj]

Dont use the isomorphic-newtype deriving trick for recursive newtypes

diff --git a/ghc/compiler/typecheck/TcDeriv.lhs b/ghc/compiler/typecheck/TcDeriv.lhs
index b072bb4..80dbea9 100644 (file)
--- a/ghc/compiler/typecheck/TcDeriv.lhs
+++ b/ghc/compiler/typecheck/TcDeriv.lhs
@@ -43,7 +43,7 @@ import RdrName                ( RdrName )
 
 import TyCon           ( tyConTyVars, tyConDataCons, tyConArity, newTyConRep,
                          tyConTheta, maybeTyConSingleCon, isDataTyCon,
-                         isEnumerationTyCon, TyCon
+                         isEnumerationTyCon, isRecursiveTyCon, TyCon
                        )
 import TcType          ( TcType, ThetaType, mkTyVarTys, mkTyConApp, getClassPredTys_maybe,
                          isUnLiftedType, mkClassPred, tyVarsOfTypes, tcSplitFunTys, 
@@ -426,6 +426,10 @@ makeDerivEqns tycl_decls
           && n_args_to_keep   >= 0             -- Well kinded: 
                                                -- eg not: newtype T a = T Int deriving( Monad )
           && eta_ok                            -- Eta reduction works
+          && not (isRecursiveTyCon tycon)      -- Does not work for recursive tycons:
+                                               --      newtype A = MkA [A]
+                                               -- Don't want
+                                               --      instance Eq [A] => Eq A !!
 
        -- Check that eta reduction is OK
        --      (a) the dropped-off args are identical