Fix error reporting for contexts during deriving (Trac 958)

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

diff --git a/compiler/typecheck/TcDeriv.lhs b/compiler/typecheck/TcDeriv.lhs
index 68c5173..90ff3a7 100644 (file)
--- a/compiler/typecheck/TcDeriv.lhs
+++ b/compiler/typecheck/TcDeriv.lhs
@@ -15,7 +15,6 @@ import DynFlags
 
 import Generics
 import TcRnMonad
-import TcMType
 import TcEnv
 import TcGenDeriv      -- Deriv stuff
 import InstEnv
@@ -763,9 +762,10 @@ solveDerivEqns overlap_flag orig_eqns
        do { let inst_tys = [mkTyConApp tc (mkTyVarTys tyvars)]
           ; theta <- addErrCtxt (derivInstCtxt1 clas inst_tys) $
                      tcSimplifyDeriv orig tc tyvars deriv_rhs
-          ; addErrCtxt (derivInstCtxt2 theta clas inst_tys) $
-            checkValidInstance tyvars theta clas inst_tys
-          ; return (sortLe (<=) theta) }       -- Canonicalise before returning the soluction
+               -- Claim: the result instance declaration is guaranteed valid
+               -- Hence no need to call:
+               --   checkValidInstance tyvars theta clas inst_tys
+          ; return (sortLe (<=) theta) }       -- Canonicalise before returning the solution
       where
        
 
@@ -995,10 +995,5 @@ derivCtxt tycon
 
 derivInstCtxt1 clas inst_tys
   = ptext SLIT("When deriving the instance for") <+> quotes (pprClassPred clas inst_tys)
-
-derivInstCtxt2 theta clas inst_tys
-  = vcat [ptext SLIT("In the derived instance declaration"),
-          nest 2 (ptext SLIT("instance") <+> sep [pprThetaArrow theta, 
-                                                 pprClassPred clas inst_tys])]
 \end{code}