Okay, that makes sense (I mistakenly thought of "didSetHasArgument" to mean "didSetHasExplicitArgument").
Review #2
The core team discussed the review feedback, which was overall very positive, and believes in principle this is a good change to accept.
During the review, some concern was expressed around how the mere use of oldValue in the body of the code would change the behavior. The core team suggested a slight amendment – that any code currently making use of an implicit oldValue receive a warning with a fixit adding it as an explicit argument.
The new proposal is going out for a short re-review and that thread can be found here.
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