Conditional sections in cabal.project can be silently ignored depending on cache state

[Bodigrim]

On two separate occasions I've hit an issue with conditionals in cabal.project, which I nevertheless cannot reliably reproduce after cabal clean. Thus this ticket is likely for future reference only, maybe someone will be more lucky / persistent.

The issue I witnessed is that

if(impl(ghc >= 9.6))

section in cabal.project was ignored by cabal build -w ghc-9.6. The necessary (but not sufficient) preconditions seem to be that ghc in PATH is ghc-9.4 and there was a successful cabal build before (implicitly with GHC 9.4).

Cf. #7783 and #8358.

[Bodigrim]

Here is a reproducer. Save

cabal-version: 3.4
name: conditionals-in-project
version: 0

library
  build-depends:
    base < 4.17

as conditionals-in-project.cabal, and

packages: .

if(impl(ghc >= 9.4))
  allow-newer: conditionals-in-project:base

as cabal.project.

Now both cabal clean && cabal build -w ghc-9.2 and cabal clean && cabal build -w ghc-9.4 succeed, but cabal clean && cabal build -w ghc-9.2 && cabal build -w ghc-9.4 fails with Could not resolve dependencies, because if(impl(ghc >= 9.4)) section is ignored.

$ cabal clean && cabal build -w ghc-9.2 && cabal build -w ghc-9.4
Resolving dependencies...
Build profile: -w ghc-9.2.5 -O1
In order, the following will be built (use -v for more details):
 - conditionals-in-project-0 (lib) (first run)
Configuring library for conditionals-in-project-0..
Preprocessing library for conditionals-in-project-0..
Building library for conditionals-in-project-0..
Resolving dependencies...
Error: cabal: Could not resolve dependencies:
[__0] trying: conditionals-in-project-0 (user goal)
[__1] next goal: base (dependency of conditionals-in-project)
[__1] rejecting: base-4.17.0.0/installed-4.17.0.0 (conflict:
conditionals-in-project => base<4.17)
[__1] skipping: base-4.17.0.0 (has the same characteristics that caused the
previous version to fail: excluded by constraint '<4.17' from
'conditionals-in-project')
[__1] rejecting: base-4.16.4.0, base-4.16.3.0, base-4.16.2.0, base-4.16.1.0,
base-4.16.0.0, base-4.15.1.0, base-4.15.0.0, base-4.14.3.0, base-4.14.2.0,
base-4.14.1.0, base-4.14.0.0, base-4.13.0.0, base-4.12.0.0, base-4.11.1.0,
base-4.11.0.0, base-4.10.1.0, base-4.10.0.0, base-4.9.1.0, base-4.9.0.0,
base-4.8.2.0, base-4.8.1.0, base-4.8.0.0, base-4.7.0.2, base-4.7.0.1,
base-4.7.0.0, base-4.6.0.1, base-4.6.0.0, base-4.5.1.0, base-4.5.0.0,
base-4.4.1.0, base-4.4.0.0, base-4.3.1.0, base-4.3.0.0, base-4.2.0.2,
base-4.2.0.1, base-4.2.0.0, base-4.1.0.0, base-4.0.0.0, base-3.0.3.2,
base-3.0.3.1 (constraint from non-upgradeable package requires installed
instance)
[__1] fail (backjumping, conflict set: base, conditionals-in-project)
After searching the rest of the dependency tree exhaustively, these were the
goals I've had most trouble fulfilling: base, conditionals-in-project

[Mikolaj]

#8819 say it resolves this issue. Could you verify and close it, if so?