It appears that clear() is not implemented for CountTableRef, despite it being available for the other 5 types of table in tables.nim. The definition of clear for CountTable looks like it was intended to cover CountTableRef as well.
I've tested that the following code:
let ct = newCountTable[string]()
fails to compile on 0.14.0.
Implement clear() for CountTableRef. Fixes #4325.