I've been trying to land the multi-pattern catch implementation from SE-0276 for awhile (https://github.com/apple/swift/pull/27776), but I've had trouble getting it reviewed since it involves fairly large AST changes that introduce frequent merge conflicts. Considering the 5.3 branch cut is coming up soon, I was hoping to get some advice on the best way to go about integrating these changes.
The best alternative to the monolithic patch I've been able to come up with so far is splitting out the swift-syntax and parser changes, and integrate those with the old AST first, diagnosing multiple clauses as parseable but not yet supported. Those changes have already been reviewed and should be good to go, but it would be less than ideal if 5.3 swift-syntax APIs gave the mistaken impression that this feature was supported, and those only account for ~10% of changes.
I wanted to see if anyone had advice on handling cross-cutting AST changes like this that require lots of updates across Parse/Sema/SILGen/LLDB.