Strangeness in interaction between record openness, optionality and subtyping

[jclark]

There is also a complication in the interaction between openness and optionality. Suppose we have

type ClosedDate record {|
   int year;
   int month;
   int day;
|};

type ExClosedDate record {|
   *ClosedDate;
   int dayOfWeek?;
|};

type OpenDate record {
   *ClosedDate;
};

type ExOpenDate record {
   *ExClosedDate;
};

Then ClosedDate is a proper subtype of ExClosedDate, but ExOpenDate is a proper subtype of OpenDate. It feels rather unintuitive that the subtyping relationship gets swapped in this way.