We incorrectly allowed some keypaths to be inferred as writable
keypaths in Swift 3/4 modes. This no longer happens when
-swift-version 5 is specified.
This warning is a limited attempt at providing some advanced notice of
code that will break, only in the cases where the keypath is a direct
argument to a keypath subscript write.
Fixes: rdar://problem/40068274
We inadvertantly allowed this in the past, so continue to do so when
compiling in Swift 3/4 mode to avoid suddenly breaking existing code.
The diagnostic here is pretty bad, and I've opened issues for that as
well as providing some kind of deprecation warning for this so that
even under Swift 3/4 mode we alert users that this is unsupported.
rdar://problem/39802797
