Build tool not rerun when build tool version changes

[andreasabel]

Reproducer using cabal-3.16.1.0 (latest release):

Step 1: using alex-3.4

cabal get java-adt-1.0.20251105
cd java-adt-*
cabal build -c 'any.alex==3.4.*'

This errors:

dist-newstyle/build/aarch64-osx/ghc-9.8.4/java-adt-1.1/x/java-adt/build/java-adt/java-adt-tmp/Lexer.hs:771:25: error: [GHC-87543]
    Ambiguous occurrence ‘fromString’.
    It could refer to
       either ‘String1.fromString’,
              imported from ‘String1’ at src/Lexer.x:3:26-35,
           or ‘GHC.Exts.fromString’,
              imported from ‘GHC.Exts’ at dist-newstyle/build/aarch64-osx/ghc-9.8.4/java-adt-1.1/x/java-adt/build/java-adt/java-adt-tmp/Lexer.hs:20:1-15
              (and originally defined in ‘Data.String’).
    |
771 | alex_action_21 = TkId . fromString
    |                         ^^^^^^^^^^

(Cf. https://hackage.haskell.org/package/java-adt-1.0.20251105/reports/2)

Step 2: using alex-3.5

cabal build -c 'any.alex==3.5.*'

Still fails with the same error.

Step 3: using alex-3.5 after cleaning

rm -rf dist-newstyle
cabal build -c 'any.alex==3.5.*'

Succeeds.

This shows that Lexer.hs is not regenerated by alex when its version is changed.
This is an unsoundness in the "recompilation" checker of cabal.