don't create a cache directory when the compiler isn't configured

[gbaz]

Resolves #8550
Resolves #8589

This is almost the same fix as #8358 but that didn't go quite far enough -- it deferred the check, but not the cache directory creation for the check.

Refs:

• cabal repl -b ... generates dist-newstyle  #8550
• cabal update creates dist-newstyle folder in $PWD #8589
• only check for compiler when project file has conditionals #8358

[Mikolaj]

@mergify rebase

[mergify]

rebase

✅ Branch has been successfully rebased

[fendor]

LGTM, no way to write a test for this I presume?

[Mikolaj]

@gbaz: would you mind adding this PR number to the changelog file for the previous PR?

[ulysses4ever]

There are talks about another 3.8 release. Should this one be backported to 3.8? I say: yes. Regardless of a release, backports are cheap, and this looks like a bugfix to me. Any thoughts?

[andreasabel]

@ulysses4ever wrote:

There are talks about another 3.8 release.

Yes, but this hasn't been decided yet, and there is no designated individual who would drive it.
The question is when backports should happen. I'd say they should happen only once it is decided that a release will happen.
I'd say a released branch (like 3.8.) should be kept release-ready, so that one can quickly publish a patched version. Release-readiness would mean that only blessed and tested changes are on the branch, and that CI is up and running. The latter seems unfortunately not the case atm. Maybe first the CI changes have to be backported to 3.8.