> On Jun 4, 2015, at 5:22 PM, Herbert Valerio Riedel <hvr@gnu.org> wrote:
> new syntax, Cabal has no way to know that a new language syntax is
> required and that thereby needs exclude (not implemented yet) the
> affected package versions from the install-plan configuration space.

I can't parse your last sentence. The proposed syntax is currently a parse error, so a package that used it could depend on a GHC new enough to support it (eg with a base version constraint). No older packages would cause any errors whatsoever.