Solver gives bad error when inplace package version incompatible with other inplace package version

[ezyang]

Steps to repro:

1. I had a local copy of hackage-server, which has a fixed dependency Cabal == 1.24.*
2. I added a dev copy of Cabal (2.1.x) to my Nix project
3. I ran the solver:

hackage@back-hackage:~/hackage-server$ cabal new-build -j                                      
Resolving dependencies...
cabal: Could not resolve dependencies:
trying: directory-1.3.0.0/installed-1.3... (dependency of Cabal-2.1.0.0)
trying: hackage-server-0.5.1 (user goal)
trying: hackage-server-0.5.1:-build-hackage-import
trying: hackage-server-0.5.1:+build-hackage-build
trying: hackage-server-0.5.1:+build-hackage-mirror
rejecting: hackage-server-0.5.1:+build-hackage-server (conflict:
directory==1.3.0.0/installed-1.3..., hackage-server-0.5.1:build-hackage-server
=> directory>=1.0 && <1.3)
rejecting: hackage-server-0.5.1:-build-hackage-server (manual flag can only be
changed explicitly)
After searching the rest of the dependency tree exhaustively, these were the
goals I've had most trouble fulfilling: hackage-server-0.5.1:flag, Cabal,
hackage-server

I don't know why it's complaining about directory. I verified that changing the bound in hackage-server solved the problem. This is probably just the "solver rejects reasonable plan early, but then doesn't emit it on default verbosity".

[grayjay]

I think this is #941 (The solver only prints the message from the first conflict, even if it isn't relevant). Another sign that the message about directory is irrelevant is that directory isn't contained in the final conflict set, on the last line. I think that my suggestion in #941 (comment) would also work here, because the conflict set is so small.

[ezyang]

Yes! And I thought this seemed familiar to me, because I had recently reported another iteration of this #4139 and forgotten I had done so. Too bad closed tickets don't show up on search. I'll close this one too.