spray/spray-json — pull request #17 by samskivert (2012-02-24 23:48:48+00:00)
Title: Made ProductFormats.extractFieldNames more robust.

The commit message gives the details.

I also threw in better error reporting, so that when you get this exception, you at least see *what* class is having the trouble.

FWIW: I encountered this issue when running under JRebel, but not when running in a straight JVM. So maybe JRebel was doing some sneaky reordering. Regardless, since the copy$default$ methods have numbers that dictate their order, we might as well use them to do the right thing.