Wibbles to instance validity checking

diff --git a/ghc/compiler/typecheck/TcMType.lhs b/ghc/compiler/typecheck/TcMType.lhs
index 6c1814e..b306d86 100644 (file)
--- a/ghc/compiler/typecheck/TcMType.lhs
+++ b/ghc/compiler/typecheck/TcMType.lhs
@@ -927,12 +927,13 @@ check_source_ty dflags ctxt sty = failWithTc (badSourceTyErr sty)
 check_class_pred_tys dflags ctxt tys 
   = case ctxt of
        TypeCtxt      -> True   -- {-# SPECIALISE instance Eq (T Int) #-} is fine
-       InstThetaCtxt -> gla_exts || all tcIsTyVarTy tys
+       InstThetaCtxt -> gla_exts || undecidable_ok || all tcIsTyVarTy tys
                                -- Further checks on head and theta in
                                -- checkInstTermination
        other         -> gla_exts || all tyvar_head tys
   where
-    gla_exts      = dopt Opt_GlasgowExts dflags
+    gla_exts       = dopt Opt_GlasgowExts dflags
+    undecidable_ok = dopt Opt_AllowUndecidableInstances dflags
 
 -------------------------
 tyvar_head ty                  -- Haskell 98 allows predicates of form 
@@ -1025,7 +1026,7 @@ checkThetaCtxt ctxt theta
          ptext SLIT("While checking") <+> pprSourceTyCtxt ctxt ]
 
 badSourceTyErr sty = ptext SLIT("Illegal constraint") <+> pprPred sty
-predTyVarErr pred  = sep [ptext SLIT("Non-type variable argument"),
+predTyVarErr pred  = sep [ptext SLIT("Non type-variable argument"),
                          nest 2 (ptext SLIT("in the constraint:") <+> pprPred pred)]
 dupPredWarn dups   = ptext SLIT("Duplicate constraint(s):") <+> pprWithCommas pprPred (map head dups)
 
@@ -1121,14 +1122,14 @@ checkValidInstance tyvars theta clas inst_tys
        -- Check that instance inference will terminate (if we care)
        -- For Haskell 98, checkValidTheta has already done that
        ; when (gla_exts && not undecidable_ok) $
-           checkInstTermination theta inst_tys
+              checkInstTermination theta inst_tys
        
        -- The Coverage Condition
        ; checkTc (undecidable_ok || checkInstCoverage clas inst_tys)
                  (instTypeErr (pprClassPred clas inst_tys) msg)
        }
   where
-    msg  = parens (ptext SLIT("the instance types do not agree with the functional dependencies of the class"))
+    msg  = parens (ptext SLIT("the Coverage Condition fails for one of the functional dependencies"))
 \end{code}
 
 Termination test: each assertion in the context satisfies