Add Travis job to build cabal with solver debug flags enabled.

[grayjay]

The job should detect issues like #3967.

[mention-bot]

@grayjay, thanks for your PR! By analyzing the history of the files in this pull request, we identified @ezyang, @phadej and @23Skidoo to be potential reviewers.

[ezyang]

Re sandbox, is this fundamentally a problem with how new-build works (i.e., would new-build work if we did it with HEAD?) Can we do something about the deps?

[grayjay]

The dependencies' setup scripts don't compile with Cabal HEAD, so we would have to force an older Cabal version. new-build at HEAD succeeds, because the dependencies lack custom-setup stanzas, and new-build adds a default setup dependency on Cabal < 1.25 (cabal-install 1.24 only constrains it to 2.0). I think that the best solution would be to use qualified constraints (#3502) in cabal.project, once they are available.

[ezyang]

Heh, oh the pains of being Cabal (maybe this behavior makes sense for other people, but it definitely doesn't make sense for us!)

[grayjay]

Yeah, Cabal definitely seems like a special case.

[phadej]

(FWIW, similar issue as compiling with -fparsec-struct-diff... cannot do that with 1.24 new-build)

[grayjay]

(FWIW, similar issue as compiling with -fparsec-struct-diff... cannot do that with 1.24 new-build)

Maybe we should apply #3939 to 1.24.

[23Skidoo]

Maybe we should apply #3939 to 1.24.

Done: 7ba7489.

[23Skidoo]

Merged, thanks!

[grayjay]

Thanks!