Improve solver error messages slightly.

As a consequence of treating all flag choices as a common goal for
conflict set computation, error message slicing was broken and did
not contain any information about flag choices anymore.

With this change, information about flag choices is once again
included in error messages.

cabal-install/Distribution/Client/Dependency/Modular/Message.hs

@@ -39,21 +39,24 @@ showMessages p sl = go [] 0
     go _ _ []                            = []
     -- complex patterns
     go v l (TryP (PI qpn i) : Enter : Failure c fr : Leave : ms) = goPReject v l qpn [i] c fr ms
-    go v l (TryF qfn b : Enter : Failure c fr : Leave : ms) = (atLevel (F qfn : v) l $ "rejecting: " ++ showQFNBool qfn b ++ showFR c fr) (go v l ms)
-    go v l (TryS qsn b : Enter : Failure c fr : Leave : ms) = (atLevel (S qsn : v) l $ "rejecting: " ++ showQSNBool qsn b ++ showFR c fr) (go v l ms)
-    go v l (Next (Goal (P qpn) gr) : TryP pi : ms@(Enter : Next _ : _)) = (atLevel (P qpn : v) l $ "trying: " ++ showPI pi ++ showGRs gr) (go (P qpn : v) l ms)
+    go v l (TryF qfn b : Enter : Failure c fr : Leave : ms) = (atLevel (add (F qfn) v) l $ "rejecting: " ++ showQFNBool qfn b ++ showFR c fr) (go v l ms)
+    go v l (TryS qsn b : Enter : Failure c fr : Leave : ms) = (atLevel (add (S qsn) v) l $ "rejecting: " ++ showQSNBool qsn b ++ showFR c fr) (go v l ms)
+    go v l (Next (Goal (P qpn) gr) : TryP pi : ms@(Enter : Next _ : _)) = (atLevel (add (P qpn) v) l $ "trying: " ++ showPI pi ++ showGRs gr) (go (add (P qpn) v) l ms)
     go v l (Failure c Backjump : ms@(Leave : Failure c' Backjump : _)) | c == c' = go v l ms
     -- standard display
     go v l (Enter                  : ms) = go v          (l+1) ms
     go v l (Leave                  : ms) = go (drop 1 v) (l-1) ms
-    go v l (TryP pi@(PI qpn _)     : ms) = (atLevel (P qpn : v) l $ "trying: " ++ showPI pi) (go (P qpn : v) l ms)
-    go v l (TryF qfn b             : ms) = (atLevel (F qfn : v) l $ "trying: " ++ showQFNBool qfn b) (go (F qfn : v) l ms)
-    go v l (TryS qsn b             : ms) = (atLevel (S qsn : v) l $ "trying: " ++ showQSNBool qsn b) (go (S qsn : v) l ms)
-    go v l (Next (Goal (P qpn) gr) : ms) = (atLevel (P qpn : v) l $ "next goal: " ++ showQPN qpn ++ showGRs gr) (go v l ms)
+    go v l (TryP pi@(PI qpn _)     : ms) = (atLevel (add (P qpn) v) l $ "trying: " ++ showPI pi) (go (add (P qpn) v) l ms)
+    go v l (TryF qfn b             : ms) = (atLevel (add (F qfn) v) l $ "trying: " ++ showQFNBool qfn b) (go (add (F qfn) v) l ms)
+    go v l (TryS qsn b             : ms) = (atLevel (add (S qsn) v) l $ "trying: " ++ showQSNBool qsn b) (go (add (S qsn) v) l ms)
+    go v l (Next (Goal (P qpn) gr) : ms) = (atLevel (add (P qpn) v) l $ "next goal: " ++ showQPN qpn ++ showGRs gr) (go v l ms)
     go v l (Next _                 : ms) = go v l ms -- ignore flag goals in the log
     go v l (Success                : ms) = (atLevel v l $ "done") (go v l ms)
     go v l (Failure c fr           : ms) = (atLevel v l $ "fail" ++ showFR c fr) (go v l ms)
 
+    add :: Var QPN -> [Var QPN] -> [Var QPN]
+    add v vs = simplifyVar v : vs
+
     -- special handler for many subsequent package rejections
     goPReject :: [Var QPN] -> Int -> QPN -> [I] -> ConflictSet QPN -> FailReason -> [Message] -> [String]
     goPReject v l qpn is c fr (TryP (PI qpn' i) : Enter : Failure _ fr' : Leave : ms) | qpn == qpn' && fr == fr' = goPReject v l qpn (i : is) c fr ms