typeclass resolution isn't working: failed is_def_eq

Stream: new members

[Sina]

I am using set_option trace.class_instances true to see why typeclass resolution isn't working in several places and the messages I get usually involve

blah blah instance
failed is_def_eq

How should I understand this? What does failed is_def_eq mean?

[Kevin Buzzard]

It means that some (possibly totally random and stupid) guess that the type class inference system tried was not correct because it turned out not to match.