[project @ 2003-11-10 12:05:04 by simonpj]

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

diff --git a/ghc/compiler/typecheck/TcSimplify.lhs b/ghc/compiler/typecheck/TcSimplify.lhs
index 7a971ab..02ed4d5 100644 (file)
--- a/ghc/compiler/typecheck/TcSimplify.lhs
+++ b/ghc/compiler/typecheck/TcSimplify.lhs
@@ -2051,8 +2051,15 @@ addNoInstanceErrs mb_what givens dicts
          | not (isClassDict dict) = (overlap_doc, dict : no_inst_dicts)
          | otherwise
          = case lookupInstEnv dflags inst_envs clas tys of
-               ([], _)  -> (overlap_doc, dict : no_inst_dicts)         -- No matches
-               inst_res -> (mk_overlap_msg dict inst_res $$ overlap_doc, no_inst_dicts)
+               res@(ms, _) 
+                 | length ms > 1 -> (mk_overlap_msg dict res $$ overlap_doc, no_inst_dicts)
+                 | otherwise     -> (overlap_doc, dict : no_inst_dicts)        -- No match
+               -- NB: there can be exactly one match, in the case where we have
+               --      instance C a where ...
+               -- (In this case, lookupInst doesn't bother to look up, 
+               --  unless -fallow-undecidable-instances is set.)
+               -- So we report this as "no instance" rather than "overlap"; the fix is
+               -- to specify -fallow-undecidable-instances, but we leave that to the programmer!
          where
            (clas,tys) = getDictClassTys dict
     in