Solver: shorten the skipping message if needed

Also add a test to prevent this regressing.

Closes: #4251

Author: erikd

cabal-install-solver/src/Distribution/Solver/Modular/Message.hs

@@ -282,16 +282,16 @@ showOption qpn@(Q _pp pn) (POption i linkedTo) =
 -- >>> showOptions foobarQPN [k1, k2]
 -- "foo-bar; foo-bar~>bazqux.foo-bar-1, foo-bar~>bazqux.foo-bar-2"
 -- >>> showOptions foobarQPN [v0, i1, k2]
--- "foo-bar; 0, 1/installed-inplace, foo-bar~>bazqux.foo-bar-2"
+-- "foo-bar; 0, 1/installed-inplace, foo-bar~>bazqux.foo-bar-2 and earlier versions"
 showOptions :: QPN -> [POption] -> String
 showOptions _ [] = "unexpected empty list of versions"
 showOptions q [x] = showOption q x
 showOptions q xs = showQPN q ++ "; " ++ (L.intercalate ", "
   [if isJust linkedTo
     then showOption q x
     else showI i -- Don't show the package, just the version
-  | x@(POption i linkedTo) <- xs
-  ])
+  | x@(POption i linkedTo) <- take 3 xs
+  ] ++ if length xs >= 3 then " and earlier versions" else "")
 
 showGR :: QGoalReason -> String
 showGR UserGoal            = " (user goal)"

cabal-install/tests/UnitTests/Distribution/Solver/Modular/Solver.hs

@@ -964,6 +964,19 @@ tests =
                   skipping = "skipping: A; 2.0.0/installed-2.0.0, 1.0.0/installed-1.0.0"
                in mkTest db "show skipping versions list, installed" ["B"] $
                     solverFailure (\msg -> rejecting `isInfixOf` msg && skipping `isInfixOf` msg)
+          , runTest $
+              let db =
+                    [ Right $ exAv "A" 1 []
+                    , Right $ exAv "A" 2 []
+                    , Right $ exAv "A" 3 []
+                    , Right $ exAv "A" 4 []
+                    , Right $ exAv "A" 5 []
+                    , Right $ exAv "B" 1 [ExFix "A" 6]
+                    ]
+                  rejecting = "rejecting: A-5.0.0 (conflict: B => A==6.0.0)"
+                  skipping = "skipping: A; 4.0.0, 3.0.0, 2.0.0 and earlier versions (has"
+               in mkTest db "show summarized skipping versions list" ["B"] $
+                    solverFailure (\msg -> rejecting `isInfixOf` msg && skipping `isInfixOf` msg)
           ]
       ]
   ]

changelog.d/pr-11062

@@ -0,0 +1,8 @@
+synopsis: Solver: shorten the skipping message if needed
+packages: cabal-install-solver
+prs: #11062
+
+When the solver fails to find a solution, it can print out a long list
+of package versions which failed to meet the requirements. This PR
+shortens the message to at most 3 versions which failed to meet the
+requriements.