Solver wants to downgrade installed package needlessly

[hvr]

@tibbe asked me to file this one. See cabal-install output at http://lpaste.net/104078 which shows the result for the following invocations:

# 1st (fails due to backjumps exhaustion)
$ cabal install aeson http-streams --dry

# 2nd (succeeds with sensible install plan)
$ cabal install http-streams aeson --dry

# 3rd (results in --force-install plan)
$ cabal install aeson http-streams --max-backjumps=500 --dry

on a current GHC 7.8.2 install with an empty (user) package database. Note, adding --constraint "bytestring installed"to the last invocation would let cabal-install find the same install-plan as for the 2nd invocation.

[hvr]

btw, here's another curious case with todays hackage (and cabal-install-1.20.0.1):

$ cabal install http-streams --dry 
Resolving dependencies...
In order, the following would be installed:
bytestring-0.10.2.0 (latest: 0.10.4.0) (new version)
base64-bytestring-1.0.0.1 (new package)
dlist-0.7.0.1 (new package)
mtl-2.1.3.1 (latest: 2.2.0.1) (new package)
primitive-0.5.2.1 (new package)
syb-0.4.1 (new package)
text-1.0.0.1 (latest: 1.1.1.2) (new package)
blaze-builder-0.3.3.2 (new package)
hashable-1.2.1.0 (new package)
case-insensitive-1.2.0.0 (new package)
parsec-3.1.5 (new package)
scientific-0.2.0.2 (new package)
attoparsec-0.11.3.0 (new package)
unix-2.7.0.1 (reinstall) changes: bytestring-0.10.4.0 -> 0.10.2.0
directory-1.2.1.0 (reinstall)
network-2.4.2.3 (latest: 2.5.0.0) (new package)
HsOpenSSL-0.10.4 (new package)
process-1.2.0.0 (reinstall)
unordered-containers-0.2.4.0 (new package)
http-common-0.7.1.1 (new package)
vector-0.10.9.1 (new package)
aeson-0.7.0.3 (new package)
zlib-0.5.4.1 (new package)
zlib-bindings-0.1.1.5 (new package)
io-streams-1.1.4.3 (new package)
openssl-streams-1.1.0.0 (new package)
http-streams-0.7.1.1 (new package)
Warning: The following packages are likely to be broken by the reinstalls:
haskell98-2.0.0.3
ghc-7.8.2
Cabal-1.18.1.3
bin-package-db-0.0.0.0
hpc-0.6.0.1
Use --force-reinstalls if you want to install anyway.

vs.

$ cabal install http-streams --constraint 'bytestring installed' --dry 
Resolving dependencies...
In order, the following would be installed (use -v for more details):
base64-bytestring-1.0.0.1
dlist-0.7.0.1
mtl-2.1.3.1 (latest: 2.2.0.1)
primitive-0.5.2.1
syb-0.4.1
text-1.1.1.2
blaze-builder-0.3.3.2
hashable-1.2.1.0
case-insensitive-1.2.0.0
parsec-3.1.5
network-2.4.2.3 (latest: 2.5.0.0)
HsOpenSSL-0.10.4
scientific-0.2.0.2
attoparsec-0.11.3.0
unordered-containers-0.2.4.0
http-common-0.7.1.1
vector-0.10.9.1
aeson-0.7.0.3
zlib-0.5.4.1
zlib-bindings-0.1.1.5
io-streams-1.1.4.3
openssl-streams-1.1.0.0
http-streams-0.7.1.1

[hvr]

@kosmikus fyi, I just re-tried the last command with the latest commit (kosmikus/cabal@a51b837) : http://lpaste.net/104324

So, if I understand it right, the problem seems to be aeson's new-bytestring-builder flag

[kosmikus]

@hvr Yes, see haskell/aeson#177. You might want to try this with kosmikus@814e493 instead (which should rebase properly on top of current master and also on top of kosmikus@a51b837).

[hvr]

@kosmikus indeed, with kosmikus/cabal@814e493 + kosmikus/cabal@a51b837 cabal install http-streams works much better

[kosmikus]

@hvr Thanks for testing. I've slightly updated the change and submitted it as pull request #1879.

[kosmikus]

Now that #1879 has been merged, I think we can close this. Feel free to reopen if you disagree.