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Cabal >= 3.0 allows set notation in the tested-with field, such as

tested-with: GHC == { 8.6.3, 8.4.4, 8.2.2, 8.0.2, 7.10.3, 7.8.4, 7.6.3, 7.4.2 }

When we use set notation in the tested-with field in package.yaml, the field is copied to the .cabal file, but the cabal-version is not set to >= 3.0 as needed. This results in the following error when we try to cabal build using the generated cabal file:

unexpected version set syntax used. To use this syntax the package needs to specify at least 'cabal-version: 3.0'. Alternatively, if broader compatibility is important then use a series of single version constraints joined with the || operator: ==8.6.3 || ==8.4.4 || ==8.2.2 || ==8.0.2 || ==7.10.3 || ==7.8.4 || ==7.6.3 || ==7.4.2

   20 | tested-with:
   21 |     GHC == { 8.6.3, 8.4.4, 8.2.2, 8.0.2, 7.10.3, 7.8.4, 7.6.3, 7.4.2 }
      |                                                                       ^                                   ^

There are two things here:

1. Should hpack accept.
2. How should hpack render it.

Regarding (1), should hpack accept something like:

tested-with:
  GHC:
    - 8.6.3
    - 8.4.4
    - ...

Regarding (2), we could still always render this as tested-with: GHC == 8.6.3, GHC == 8.4.4, ... for maximum compatibility. Or is there a reason not to?

Finally, for your original request, when tested-with is given as a string, then I'm not eager to look at it and pass it through transparently. In that case it would be the responsibility of the user to adjust the cabal-version as needed.

Regarding (1), that seems reasonable to me to accept. Regarding (2), that also seems like a reasonable way to render it. It also makes sense to just pass through tested-with transparently when it's passed as a string.

Also, as I think about it more, I think the current option of writing

tested-with:
  - GHC == 8.6.3
  - GHC == 8.4.4
  - ...

seems like an acceptable workaround (though a little less aesthetically pleasing).

Regarding (1), that seems reasonable to me to accept.

PRs welcome.