Replace <0 with some other construct to avoid confusion

[jasagredo]

Having a bound <0 means that it is unsatisfiable, it is not a real bound as it cannot be influenced by --allow-newer:

➜ cat aa.cabal
cabal-version:   3.0
name:            aa
version:         0.1.0.0
license:         NONE
build-type:      Simple
extra-doc-files: CHANGELOG.md

common warnings
    ghc-options: -Wall

library
    import:           warnings
    exposed-modules:  MyLib
    build-depends:    base <0
    hs-source-dirs:   src
    default-language: Haskell2010

➜ cabal build --allow-newer
Resolving dependencies...
Error: [Cabal-7107]
Could not resolve dependencies:
[__0] trying: aa-0.1.0.0 (user goal)
[__1] next goal: base (dependency of aa)
[__1] rejecting: base-4.18.2.1/installed-4.18.2.1 (conflict: aa => base<0)
[__1] skipping: base; 4.20.0.1, 4.20.0.0, 4.19.1.0, 4.19.0.0, 4.18.2.1, 4.18.2.0, 4.18.1.0, 4.18.0.0, 4.17.2.1, 4.17.2.0, 4.17.1.0, 4.17.0.0, 4.16.4.0, 4.16.3.0, 4.16.2.0, 4.16.1.0, 4.16.0.0, 4.15.1.0, 4.15.0.0, 4.14.3.0, 4.14.2.0, 4.14.1.0, 4.14.0.0, 4.13.0.0, 4.12.0.0, 4.11.1.0, 4.11.0.0, 4.10.1.0, 4.10.0.0, 4.9.1.0, 4.9.0.0, 4.8.2.0, 4.8.1.0, 4.8.0.0, 4.7.0.2, 4.7.0.1, 4.7.0.0, 4.6.0.1, 4.6.0.0, 4.5.1.0, 4.5.0.0, 4.4.1.0, 4.4.0.0, 4.3.1.0, 4.3.0.0, 4.2.0.2, 4.2.0.1, 4.2.0.0, 4.1.0.0, 4.0.0.0, 3.0.3.2, 3.0.3.1 (has the same characteristics that caused the previous version to fail: excluded by constraint '<0' from 'aa')
[__1] fail (backjumping, conflict set: aa, base)
After searching the rest of the dependency tree exhaustively, these were the goals I've had most trouble fulfilling: aa, base

➜ sed -i 's/<0/<0.0/g' aa.cabal

➜ cabal build --allow-newer
Resolving dependencies...
Build profile: -w ghc-9.6.5 -O1
In order, the following will be built (use -v for more details):
 - aa-0.1.0.0 (lib) (first run)
Configuring library for aa-0.1.0.0...

I think it would be good if we used some other construct that what looks like a normal bound when we want something to be unbuildable.

[jasagredo]

FTR, Cabal 1.22 introduced the -none version:

➜ cat aa.cabal
cabal-version:   3.0
name:            aa
version:         0.1.0.0
license:         NONE
build-type:      Simple
extra-doc-files: CHANGELOG.md

common warnings
    ghc-options: -Wall

library
    import:           warnings
    exposed-modules:  MyLib
    build-depends:    base -none
    hs-source-dirs:   src
    default-language: Haskell2010

This would be precisely the construct I'm talking about. However -none results in the range <0 which is still considered unsat because noVersion = EarlierVersion (mkVersion [0]).

[andreabedini]

The code path is

-- in Distribution.Client.Dependency

removeUpperBounds :: AllowNewer -> DepResolverParams -> DepResolverParams

removeBounds :: RelaxKind -> RelaxDeps -> DepResolverParams -> DepResolverParams

relaxPackageDeps
  :: RelaxKind
  -> RelaxDeps
  -> PD.GenericPackageDescription
  -> PD.GenericPackageDescription

removeBound :: RelaxKind -> RelaxDepMod -> VersionRange -> VersionRange

-- in Distribution.Version

-- | Given a version range, remove the highest upper bound. Example: @(>= 1 && <
-- 3) || (>= 4 && < 5)@ is converted to @(>= 1 && < 3) || (>= 4)@.
removeUpperBound :: VersionRange -> VersionRange
removeUpperBound = fromVersionIntervals . relaxLastInterval . toVersionIntervals

-- | Given a version range, remove the lowest lower bound.
-- Example: @(>= 1 && < 3) || (>= 4 && < 5)@ is converted to
-- @(>= 0 && < 3) || (>= 4 && < 5)@.
removeLowerBound :: VersionRange -> VersionRange
removeLowerBound = fromVersionIntervals . relaxHeadInterval . toVersionIntervals

and indeed

ghci> noVersion
EarlierVersion (mkVersion [0])
ghci> removeUpperBound noVersion
EarlierVersion (mkVersion [0])

I guess the logic bug is in toVersionIntervals:

ghci> toVersionIntervals noVersion
VersionIntervals []