cubicweb #2017735 Entity type facet crash when initial query uses "is_instance_of" [open]

Imagine you have a base Fruit etype, with Apple and Grape specializing it (yams inheritance).

Then, if you view all Fruit instances ("Any X WHERE X is_instance_of Fruit"), further refining your rset by selecting the Grape entity type facet, you get a rql query like "Any X WHERE X is_instance_of Fruit, X is Grape", which is currently illegal (error message is : "can only one type restriction per variable (use IN for X if desired)")

Note I am unsure if RQL should accept such a query or if it is to be considered a facet bug.

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