If dependent (child) CL is CQ-ed *before* its dependency (parent) CL lands, when CQ calls REST api "submit", Gerrit may reply with this:

Failed to submit 1 change due to the following problems:
Change 392207: Change could not be merged due to a path conflict. Please rebase the change locally and upload the rebased commit for review.

(example: https://chromium-review.googlesource.com/c/392207/)

But other times, Gerrit would complain that parent CL hasn't landed, which is better. IMO, CQ should just check for parent Cls, if any, and abort *right before issuing Submit call*.

This confuses users. For Rietveld, CQ would abort before even scheduling tryjobs in this situation.