Ok, I don't know why I didn't notice this before... but it's not over yet.
Ouch.
There is another -- very similar -- issue that causes trouble if there are EQL method specialized on the system in the .asd file for the system in question ... not exactly a rarity.
Try with ITERATE. It recurses endlessly with 2.015.2 between local installation and Quicklisp one.
The proximate casue is (FIND-SYSTEM :iterate): when the non-quicklisp .asd is processed, those methods find the Quicklisp definitions, and vice versa.
OK. In ASDF 2.015.3, I've built some more robust plumbing to prevent infinite recursion in finding systems. Guaranteed to not recurse infinitely while searching for systems if your systems are loaded through find-system (or its internal load-sysdef) or operate (and its wrappers oos, load-system, compile-system).
Can you test again? Someone contributing a test case for ASDF would be nice. I'm too lazy/busy to do it right yet.
—♯ƒ • François-René ÐVB Rideau •Reflection&Cybernethics• http://fare.tunes.org Quis custodiet ipsos custodes? (Who shall watch the watchmen themselves?) — Juvenal, Satires, VI, 347