import DynFlags
import StaticFlags( opt_PprStyle_Debug )
import Data.List( partition )
-import Control.Monad( unless )
+import Control.Monad( when, unless )
\end{code}
%************************************************************************
reportTidyWanteds :: ReportErrCtxt -> WantedConstraints -> TcM ()
reportTidyWanteds ctxt unsolved
- = do { let (flats, implics) = splitWanteds unsolved
- (ambigs, others) = partition is_ambiguous (bagToList flats)
- ; groupErrs (reportFlat ctxt) others
- ; mapBagM_ (reportTidyImplic ctxt) implics
- ; ifErrsM (return ()) $
- -- Only report ambiguity if no other errors happened
- -- See Note [Avoiding spurious errors]
- reportAmbigErrs ctxt skols ambigs }
+ = do { let (flats, implics) = splitWanteds unsolved
+ (ambigs, non_ambigs) = partition is_ambiguous (bagToList flats)
+ (tv_eqs, others) = partition is_tv_eq non_ambigs
+
+ ; groupErrs (reportEqErrs ctxt) tv_eqs
+ ; when (null tv_eqs) $ groupErrs (reportFlat ctxt) others
+ ; when (null tv_eqs) $ mapBagM_ (reportTidyImplic ctxt) implics
+
+ -- Only report ambiguity if no other errors (at all) happened
+ -- See Note [Avoiding spurious errors] in TcSimplify
+ ; ifErrsM (return ()) $ reportAmbigErrs ctxt skols ambigs }
where
skols = foldr (unionVarSet . ic_skols) emptyVarSet (cec_encl ctxt)
+ -- Report equalities of form (a~ty) first. They are usually
+ -- skolem-equalities, and they cause confusing knock-on
+ -- effects in other errors; see test T4093b.
+ is_tv_eq c | EqPred ty1 ty2 <- wantedEvVarPred c
+ = tcIsTyVarTy ty1 || tcIsTyVarTy ty2
+ | otherwise = False
+
-- Treat it as "ambiguous" if
-- (a) it is a class constraint
-- (b) it constrains only type variables
; wrapErrTcS $ setCtFlavorLoc fl $
do { -- Apply the current substitition
-- and zonk to get rid of flatten-skolems
- ; ty_binds_bag <- readTcRef ty_binds_var
- ; let subst = mkOpenTvSubst (mkVarEnv (bagToList ty_binds_bag))
+ ; ty_binds_map <- readTcRef ty_binds_var
+ ; let subst = mkOpenTvSubst (mapVarEnv snd ty_binds_map)
; env0 <- tcInitTidyEnv
; (env1, ty1) <- zonkSubstTidy env0 subst ty1
; (env2, ty2) <- zonkSubstTidy env1 subst ty2