Improve consistency checking for derived instances

This patch arranges that derived instances use the same instance-decl
checking code as user-defined instances.  That gives greater consistency
in error messages.

Furthermore, the error description if this consistency check fails is now
much more explicit.  For example, drvfail003 now says
     Variable occurs more often in a constraint than in the instance head
       in the constraint: Show (v (v a))
     (Use -fallow-undecidable-instances to permit this)
     In the derived instance
       instance (Show (v (v a))) => Show (Square_ v w a)

diff --git a/compiler/typecheck/TcDeriv.lhs b/compiler/typecheck/TcDeriv.lhs
index 1f4c476..95d9697 100644 (file)
--- a/compiler/typecheck/TcDeriv.lhs
+++ b/compiler/typecheck/TcDeriv.lhs
@@ -15,6 +15,7 @@ import DynFlags       ( DynFlag(..) )
 
 import Generics                ( mkTyConGenericBinds )
 import TcRnMonad
+import TcMType         ( checkValidInstance )
 import TcEnv           ( newDFunName, pprInstInfoDetails, 
                          InstInfo(..), InstBindings(..), simpleInstInfoClsTy,
                          tcLookupClass, tcLookupTyCon, tcExtendTyVarEnv
@@ -30,7 +31,7 @@ import RnEnv          ( bindLocalNames )
 import HscTypes                ( FixityEnv )
 
 import Class           ( className, classArity, classKey, classTyVars, classSCTheta, Class )
-import Type            ( zipOpenTvSubst, substTheta )
+import Type            ( zipOpenTvSubst, substTheta, pprThetaArrow, pprClassPred )
 import ErrUtils                ( dumpIfSet_dyn )
 import MkId            ( mkDictFunId )
 import DataCon         ( isNullarySrcDataCon, isVanillaDataCon, dataConOrigArgTys )
@@ -341,7 +342,7 @@ makeDerivEqns overlap_flag tycl_decls
     mk_eqn (new_or_data, tycon_name, hs_deriv_ty)
       = tcLookupTyCon tycon_name               `thenM` \ tycon ->
        setSrcSpan (srcLocSpan (getSrcLoc tycon))               $
-        addErrCtxt (derivCtxt Nothing tycon)   $
+        addErrCtxt (derivCtxt tycon)           $
        tcExtendTyVarEnv (tyConTyVars tycon)    $       -- Deriving preds may (now) mention
                                                        -- the type variables for the type constructor
        tcHsDeriv hs_deriv_ty                   `thenM` \ (deriv_tvs, clas, tys) ->
@@ -726,10 +727,15 @@ solveDerivEqns overlap_flag orig_eqns
 
     ------------------------------------------------------------------
     gen_soln (_, clas, tc,tyvars,deriv_rhs)
-      = setSrcSpan (srcLocSpan (getSrcLoc tc))         $
-       addErrCtxt (derivCtxt (Just clas) tc)   $
-       tcSimplifyDeriv tc tyvars deriv_rhs     `thenM` \ theta ->
-       returnM (sortLe (<=) theta)     -- Canonicalise before returning the soluction
+      = setSrcSpan (srcLocSpan (getSrcLoc tc)) $
+       do { let inst_tys = [mkTyConApp tc (mkTyVarTys tyvars)]
+          ; theta <- addErrCtxt (derivInstCtxt [] clas inst_tys) $
+                     tcSimplifyDeriv tc tyvars deriv_rhs
+          ; addErrCtxt (derivInstCtxt theta clas inst_tys) $
+            checkValidInstance tyvars theta clas inst_tys
+          ; return (sortLe (<=) theta) }       -- Canonicalise before returning the soluction
+      where
+       
 
     ------------------------------------------------------------------
     mk_inst_spec (dfun_name, clas, tycon, tyvars, _) theta
@@ -950,12 +956,12 @@ derivingThingErr clas tys tycon tyvars why
   where
     pred = mkClassPred clas (tys ++ [mkTyConApp tycon (mkTyVarTys tyvars)])
 
-derivCtxt :: Maybe Class -> TyCon -> SDoc
-derivCtxt maybe_cls tycon
-  = ptext SLIT("When deriving") <+> cls <+> ptext SLIT("for type") <+> quotes (ppr tycon)
-  where
-    cls = case maybe_cls of
-           Nothing -> ptext SLIT("instances")
-           Just c  -> ptext SLIT("the") <+> quotes (ppr c) <+> ptext SLIT("instance")
+derivCtxt :: TyCon -> SDoc
+derivCtxt tycon
+  = ptext SLIT("When deriving instances for") <+> quotes (ppr tycon)
+
+derivInstCtxt theta clas inst_tys
+  = hang (ptext SLIT("In the derived instance"))
+       2 (ptext SLIT("instance") <+> sep [pprThetaArrow theta, pprClassPred clas inst_tys])
 \end{code}
 

diff --git a/compiler/typecheck/TcSimplify.lhs b/compiler/typecheck/TcSimplify.lhs
index 19557c6..4cb32b8 100644 (file)
--- a/compiler/typecheck/TcSimplify.lhs
+++ b/compiler/typecheck/TcSimplify.lhs
@@ -42,8 +42,7 @@ import Inst           ( lookupInst, LookupInstResult(..),
 import TcEnv           ( tcGetGlobalTyVars, tcLookupId, findGlobals, pprBinders,
                          lclEnvElts, tcMetaTy )
 import InstEnv         ( lookupInstEnv, classInstances, pprInstances )
-import TcMType         ( zonkTcTyVarsAndFV, tcInstTyVars, zonkTcPredType,
-                         checkAmbiguity, checkInstTermination )
+import TcMType         ( zonkTcTyVarsAndFV, tcInstTyVars, zonkTcPredType  )
 import TcType          ( TcTyVar, TcTyVarSet, ThetaType, TcPredType, tidyPred,
                           mkClassPred, isOverloadedTy, mkTyConApp, isSkolemTyVar,
                          mkTyVarTy, tcGetTyVar, isTyVarClassPred, mkTyVarTys,
@@ -2283,19 +2282,10 @@ tcSimplifyDeriv tc tyvars theta
        rev_env = zipTopTvSubst tvs (mkTyVarTys tyvars)
                -- This reverse-mapping is a Royal Pain, 
                -- but the result should mention TyVars not TcTyVars
-
-       head_ty = TyConApp tc (map TyVarTy tvs)
     in
    
     addNoInstanceErrs Nothing [] bad_insts             `thenM_`
     mapM_ (addErrTc . badDerivedPred) weird_preds      `thenM_`
-    checkAmbiguity tvs simpl_theta tv_set              `thenM_`
-      -- Check instance termination as for user-declared instances.
-      -- unless we had -fallow-undecidable-instances (which risks
-      -- non-termination in the 'deriving' context-inference fixpoint
-      -- loop).
-    ifM (gla_exts && not undecidable_ok)
-       (checkInstTermination simpl_theta [head_ty])    `thenM_`
     returnM (substTheta rev_env simpl_theta)
   where
     doc    = ptext SLIT("deriving classes for a data type")