Increase `max-backjumps` default from 2000 to 4000

The last time the max-backjumps was increased was back in 2014
(see 0229cd52) as suggested
per #1780, and while `--reorder-goals` might often help as well,
it also often comes at a bigger runtime penalty on average than
doubling the max-backjumps.

This should also address issues such as #5882

Cabal/doc/nix-local-build.rst

@@ -2050,13 +2050,13 @@ Most users generally won't need these.
                --max-backjumps=N
     :synopsis: Maximum number of solver backjumps.
 
-    :default: 2000
+    :default: 4000
 
     Maximum number of backjumps (backtracking multiple steps) allowed
     while solving. Set -1 to allow unlimited backtracking, and 0 to
     disable backtracking completely.
 
-    The command line variant of this field is ``--max-backjumps=2000``.
+    The command line variant of this field is ``--max-backjumps=4000``.
 
 .. cfg-field:: reorder-goals: boolean
                --reorder-goals

cabal-install/Distribution/Client/Setup.hs

@@ -1781,7 +1781,7 @@ defaultInstallFlags = InstallFlags {
                                    </> "$arch-$os-$compiler" </> "index.html")
 
 defaultMaxBackjumps :: Int
-defaultMaxBackjumps = 2000
+defaultMaxBackjumps = 4000
 
 defaultSolver :: PreSolver
 defaultSolver = AlwaysModular

cabal-install/changelog

@@ -19,6 +19,7 @@
 	* Init improvements: add flag '--application-dir', and when creating
 	  a library also create a MyLib.hs module. (#5740)
 	* Add support for generating test-suite via cabal init. (#5761)
+	* Increase `max-backjumps` default from 2000 to 4000.
 
 2.4.1.0 Mikhail Glushenkov <mikhail.glushenkov@gmail.com> November 2018
 	* Add message to alert user to potential package casing errors. (#5635)