Reported by Hubert Tong to the -edit reflector:

The wording under Stage 2 in N3937 subclause 22.4.2.1.2 [facet.num.get.virtuals] paragraph 3 (in p3.4) which reads

if the conversion function fails to convert the entire field

leaves open the possibility that there is no "failure" if the subject sequence (as defined by C) is empty since no attempt at conversion occurs in such a case. I believe that modifying the wording to "if the conversion function does not convert the entire field" is a purely editorial change which addresses the issue.