Dependency solver allows cycles between packages and their setup scripts.

[grayjay]

I noticed this while debugging #4154.

example.cabal:

name: example
version: 1
cabal-version: >= 1.2

custom-setup
  setup-depends:
    example

library

$ cabal new-build --dry-run -v3
[...]
Resolving dependencies...
targets: example
constraints: 
  base installed (non-upgradeable package)
  ghc-prim installed (non-upgradeable package)
  integer-gmp installed (non-upgradeable package)
  Cabal >=1.20 (new-build's support of Custom Setup (issue #3932))
  example ==1 (user target)
preferences: 
  example [TestStanzas,BenchStanzas]
  ADPfusion <0.4.0.0 || >0.4.0.0
  [...]
  yocto >=1
  zip-archive <0.2.3 || >0.2.3 && <0.2.3.1 || >0.2.3.1
strategy: PreferLatestForSelected
reorder goals: False
count conflicts: True
independent goals: False
avoid reinstalls: False
shadow packages: False
strong flags: False
max backjumps: 2000
[__0] trying: example-1 (user goal)
[__1] next goal: example-setup.example (dependency of example-1)
[__1] trying: example-setup.example~>example-1
[__2] done
internal error: could not construct a valid install plan.
The proposed (invalid) plan contained the following problems:
Package example-1 has an invalid configuration, in particular:
the package has a dependency example -any but no package has been selected to
satisfy it.

Proposed plan:
Configured example-1

CallStack (from HasCallStack):
error, called at ./Distribution/Client/Dependency.hs:705:33 in
main:Distribution.Client.Dependency

The solver should keep searching, because it might be able to avoid the cycle in some situations.