The function `eieio--known-slot-name-p' is used by some eieio functions
(specifically eieio-oref, eieio-oref-default, and eieio-oset-default) to
check whether a slot name is known or not, and if not signals an
"Unknown slot" warning.

Structs do not make their slots known to `eieio--known-slot-name-p', so
the warning is signaled even for valid slot names.  Since eieio aims for
compatibility with defstruct, this should probably be considered a bug.

The function `eieio--known-slot-name-p' checks the variable
`eieio--known-slot-names' but also has a fallback for the name's
`slot-name' property.  In the patch I opted to use the latter.