Annex E (informative) Conformance with UAX #31 [uaxid]

E.2 R1 Default identifiers [uaxid.def]

E.2.2 R1a Restricted format characters [uaxid.def.rfmt]

If an implementation of UAX #31 wishes to allow format characters such as ZERO WIDTH JOINER or ZERO WIDTH NON-JOINER it must define a profile allowing them, or describe precisely which combinations are permitted.
C++ does not allow format characters in identifiers, so this does not apply.