cabal-version check doesn't happen early enough

[ezyang]

With https://github.com/danidiaz/wl-pprint-indef/tree/problemo when built you get the error:

Error:
    Mix-in refers to non-existent package 'wl-pprint-indef'
    (did you forget to add the package to build-depends?)
    In the stanza 'executable wl-pprint-string-example'
    In the inplace package 'wl-pprint-indef-1.2'

But everything exists. In fact, the problem is that cabal-version is not set high enough to get the correct semantics in this case.

I'm not sure if the correct fix is to move cabal-version sanity checking earlier, or to add a little extra logic to the cabal-version dependent bits of mixin linking to give a warning when something has gone wrong, but something is worth doing here.

[hvr]

Is this something that can be handled in cabal check? or can this be even caught by the parser? i.e. can this be detected statically by looking at the .cabal file?

/cc @phadej

[phadej]

New parser framework has since annotations. They just don't do anything right now, I'll check this soln

[phadej]

Fixed in #4971 / #4953. mixins / signature fields are warned about if not cabal-version: 2.0 or later

[ezyang]

Thank you @phadej! You rock.