unable to list source for <exception thrown> should never occur

I get the very unhelpful "unable to list source for <exception thrown>". I would like to get one of the following two responses, the last one is best.

Do this and that to list source

We currently cannot list source, because you did and that Do you still want to list source                although it requires to do that and that(for example automatically recompiling and reexecuting it until the same program point) [Y/n]?

Trac metadata

Trac field	Value
Version	6.8.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system	Unknown
Architecture	Unknown