Fix Trac #3261: make default types play nice with -Werror

The trial-and-error for type defaults was not playing nicely with
-Werror. The fix is simple.

diff --git a/compiler/typecheck/TcSimplify.lhs b/compiler/typecheck/TcSimplify.lhs
index 7aaa1ae..cf29748 100644 (file)
--- a/compiler/typecheck/TcSimplify.lhs
+++ b/compiler/typecheck/TcSimplify.lhs
@@ -2903,12 +2903,16 @@ disambigGroup :: [Type]                 -- The default types
              -> TcM () -- Just does unification, to fix the default types
 
 disambigGroup default_tys dicts
-  = try_default default_tys
+  = do { mb_chosen_ty <- try_default default_tys
+       ; case mb_chosen_ty of
+            Nothing        -> return ()
+            Just chosen_ty -> do { unifyType chosen_ty (mkTyVarTy tyvar) 
+                                ; warnDefault dicts chosen_ty } }
   where
     (_,_,tyvar) = ASSERT(not (null dicts)) head dicts  -- Should be non-empty
     classes = [c | (_,c,_) <- dicts]
 
-    try_default [] = return ()
+    try_default [] = return Nothing
     try_default (default_ty : default_tys)
       = tryTcLIE_ (try_default default_tys) $
        do { tcSimplifyDefault [mkClassPred clas [default_ty] | clas <- classes]
@@ -2918,10 +2922,7 @@ disambigGroup default_tys dicts
                -- For example, if Real a is reqd, but the only type in the
                -- default list is Int.
 
-               -- After this we can't fail
-          ; warnDefault dicts default_ty
-          ; unifyType default_ty (mkTyVarTy tyvar) 
-          ; return () -- TOMDO: do something with the coercion
+          ; return (Just default_ty) -- TOMDO: do something with the coercion
           }