No access to fields on old an new value in a typePolicy.merge function

[phryneas]

Issue Description

This came up here on Discord:

If an object is overwritten with a new object of the same id, both incoming and existing are ref objects with the same value, so readField will return the same value for both incoming and existing refs. The Discord user reported this to be the value of existing, in my reproduction it's the value of incoming.

This doesn't seem something that can easily be fixed and would either require a different kind of ref, or an api change (readExistingField? 😒 ).

Not sure what exactly the plan should be here, but I thought I'd create an issue so we can track it, as this seems like a valid use case.

Link to Reproduction

41b4c45

Reproduction Steps

See the test.

[jerelmiller]

Related: https://community.apollographql.com/t/how-to-get-the-old-item-value-before-it-is-overwritten-by-the-subscription/6984

[mogelbrod]

Encountered this today, and only found this after a couple of hours of debugging and googling. Given that existing and incoming are named they way they are it's very confusing to not be able to access the old data via existing.