Fix Trac #2307: need to nub bad fundep reports

diff --git a/compiler/types/FunDeps.lhs b/compiler/types/FunDeps.lhs
index 949cac4..69533dc 100644 (file)
--- a/compiler/types/FunDeps.lhs
+++ b/compiler/types/FunDeps.lhs
@@ -29,6 +29,7 @@ import Outputable
 import Util
 import FastString
 
+import Data.List       ( nubBy )
 import Data.Maybe      ( isJust )
 \end{code}
 
@@ -469,7 +470,8 @@ badFunDeps :: [Instance] -> Class
           -> TyVarSet -> [Type]        -- Proposed new instance type
           -> [Instance]
 badFunDeps cls_insts clas ins_tv_set ins_tys 
-  = [ ispec | fd <- fds,       -- fds is often empty
+  = nubBy eq_inst $
+    [ ispec | fd <- fds,       -- fds is often empty, so do this first!
              let trimmed_tcs = trimRoughMatchTcs clas_tvs fd rough_tcs,
              ispec@(Instance { is_tcs = inst_tcs, is_tvs = tvs, 
                                is_tys = tys }) <- cls_insts,
@@ -482,6 +484,13 @@ badFunDeps cls_insts clas ins_tv_set ins_tys
   where
     (clas_tvs, fds) = classTvsFds clas
     rough_tcs = roughMatchTcs ins_tys
+    eq_inst i1 i2 = instanceDFunId i1 == instanceDFunId i2
+       -- An single instance may appear twice in the un-nubbed conflict list
+       -- because it may conflict with more than one fundep.  E.g.
+       --      class C a b c | a -> b, a -> c
+       --      instance C Int Bool Bool
+       --      instance C Int Char Char
+       -- The second instance conflicts with the first by *both* fundeps
 
 trimRoughMatchTcs :: [TyVar] -> FunDep TyVar -> [Maybe Name] -> [Maybe Name]
 -- Computing rough_tcs for a particular fundep