Resolving dependencies when constraint can't be found, maybe update?

[philderbeast]

$ cabal build squeal-postgresql --enable-tests --enable-benchmarks --minimize-conflict-set
Resolving dependencies...
Error: cabal: Could not resolve dependencies:
[__0] trying: my-package (user goal)
[__1] next goal: generics-sop (dependency of my-package)
[__1] rejecting: generics-sop-0.5.1.2, generics-sop-0.5.1.1,
generics-sop-0.5.1.0, generics-sop-0.5.0.0, generics-sop-0.4.0.1,
generics-sop-0.3.2.0, generics-sop-0.3.1.0, generics-sop-0.3.0.0,
generics-sop-0.2.5.0, generics-sop-0.2.4.0, generics-sop-0.2.3.0,
generics-sop-0.2.2.0, generics-sop-0.2.1.0, generics-sop-0.2.0.0,
generics-sop-0.1.1.2, generics-sop-0.1.1.1, generics-sop-0.1.1,
generics-sop-0.1.0.4, generics-sop-0.1.0.3, generics-sop-0.1.0.2,
generics-sop-0.1.0.1, generics-sop-0.1.0.0, generics-sop-0.4.0.0 (constraint
from project config https://www.stackage.org/nightly-2023-04-25/cabal.config requires
==0.5.1.3)
[__1] fail (backjumping, conflict set: my-package, generics-sop)
After searching the rest of the dependency tree exhaustively, these were the
goals I've had most trouble fulfilling: generics-sop, my-package

There's nothing in this message that hints at the real problem (that I expect is known to cabal), where to find generics-sop-0.5.1.3? Turns out this is fixed with cabal update but that is not suggested by the message. Could we improve this message?

[Mikolaj]

That's a very sane idea. I'd be surprised if it's not proposed already, but I haven't searched.

[philderbeast]

"One improvement to this proposal would be to mark the latest known version of a package to the solver" from #8939 would also help here. Concretely, if cabal said it knew of generics-sop-0.5.1.2 (the latest version known to cabal) but had never heard of generics-sop-0.5.1.3 then (even without cabal suggesting a cabal update) we could guess something is not quite right. As it stands there's nothing that distinguishes generics-sop-0.5.1.3 from the rest (other than not being included in the reject list).

A more helpful message could read something like:

Error: cabal: Could not resolve dependencies:
[__0] trying: my-package (user goal)
[__1] next goal: generics-sop (dependency of my-package)
[__1] unknown: generics-sop-0.5.1.3 (have you tried cabal update?)
[__1] rejecting: generics-sop-0.5.1.2 (the latest known version) and all earlier versions
(constraint from project config https://www.stackage.org/nightly-2023-04-25/cabal.config requires ==0.5.1.3)
[__1] fail (backjumping, conflict set: my-package, generics-sop)
After searching the rest of the dependency tree exhaustively, these were the
goals I've had most trouble fulfilling: generics-sop, my-package

[grayjay]

The idea for simplifying the error message is very similar to issue #4251.