Solver failure on 3.0 with distsdir from 2.4.1.0 due to outdated "compiler" cache

[DanielG]

Describe the bug

When running v2-build from the current 3.0 branch in a project with any other-extensions declared and where the project was last built with cabal-install-2.4.1.0 a solver failure will occur.

It seems to me the dist-newstyle/cache/compiler cache is not getting invalidated even though the new cabal version expects more info to be in there. If I remove that file everything is fine.

To Reproduce
Steps to reproduce the behavior:

$ cabal init -m -n -p repro --is-executable
$ echo '  other-extensions: NondecreasingIndentation' >> repro.cabal

$ rm -r dist-newstyle
$ cabal-2.4.1.0 v2-build

$ cabal-3.0.0.0 v2-build # solver failure
Warning: Parsing the index cache failed (Unknown encoding for constructor).
Trying to regenerate the index cache...
Resolving dependencies...
cabal: Could not resolve dependencies:
[__0] next goal: repro (user goal)
[__0] rejecting: repro-0.1.0.0 (conflict: requires NondecreasingIndentation)
[__0] fail (backjumping, conflict set: repro)
After searching the rest of the dependency tree exhaustively, these were the
goals I've had most trouble fulfilling: repro

$ rm dist-newstyle/cache/compiler
$ cabal-3.0.0.0 v2-build # works

System informataion

• OS: Debian buster
• ghc version: 8.4.4

[DanielG]

Seems this might actually be a problem with the NondecreasingIndentation extension specifically or at least some subset of options. DeriveFunction doesn't trigger this but ImpredicativeTypes also does.

My guess would be 3f497f2 is related though.

[TomMD]

I've seen similar behavior but with ViewPatterns:

[__0] trying: analysis-data-0.1.0.0 (user goal)
[__1] trying: turtle-1.5.14 (dependency of analysis-data)
[__2] next goal: clock (dependency of turtle)
[__2] rejecting: clock-0.8 (conflict: requires ViewPatterns)
[__2] rejecting: clock-0.7.2, clock-0.7.1.1, clock-0.7.1, clock-0.6.0.1,
clock-0.6.0, clock-0.5.2, clock-0.5.1, clock-0.5.0.1, clock-0.5.0.0,
clock-0.4.6.0, clock-0.4.5.0, clock-0.4.4.0, clock-0.4.3.0, clock-0.4.2.0,
clock-0.4.1.3, clock-0.4.1.2, clock-0.4.1.1, clock-0.4.0.1, clock-0.3,
clock-0.2.0.0, clock-0.1.2.1, clock-0.1.2, clock-0.1.1, clock-0.1 (constraint
from project config TODO requires ==0.8)
[__2] fail (backjumping, conflict set: clock, turtle)

This is when using cabal-install 3.0.0.0 and new-test on a project that was previously driven using cabal 2.4.1.0.

[hvr]

Duplicate of #6099

(see specifically my comment #6099 (comment) on how to fix this)

[23Skidoo]

Closing in favour of #6099.