Improve error message (Trac #1446)

diff --git a/compiler/typecheck/TcSimplify.lhs b/compiler/typecheck/TcSimplify.lhs
index 3f25a4c..773b312 100644 (file)
--- a/compiler/typecheck/TcSimplify.lhs
+++ b/compiler/typecheck/TcSimplify.lhs
@@ -2716,11 +2716,11 @@ report_no_instances tidy_env mb_what insts
            (clas,tys) = getDictClassTys wanted
 
     mk_overlap_msg dict (matches, unifiers)
-      = vcat [ addInstLoc [dict] ((ptext SLIT("Overlapping instances for") 
+      = ASSERT( not (null matches) )
+        vcat [ addInstLoc [dict] ((ptext SLIT("Overlapping instances for") 
                                        <+> pprPred (dictPred dict))),
                sep [ptext SLIT("Matching instances") <> colon,
                     nest 2 (vcat [pprInstances ispecs, pprInstances unifiers])],
-               ASSERT( not (null matches) )
                if not (isSingleton matches)
                then    -- Two or more matches
                     empty
@@ -2728,7 +2728,8 @@ report_no_instances tidy_env mb_what insts
                ASSERT( not (null unifiers) )
                parens (vcat [ptext SLIT("The choice depends on the instantiation of") <+>
                                 quotes (pprWithCommas ppr (varSetElems (tyVarsOfInst dict))),
-                             ptext SLIT("Use -fallow-incoherent-instances to use the first choice above")])]
+                             ptext SLIT("To pick the first instance above, use -fallow-incoherent-instances"),
+                             ptext SLIT("when compiling the other instances")])]
       where
        ispecs = [ispec | (ispec, _) <- matches]