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

E.2 R1 Default identifiers [uaxid.def]

E.2.3 R1b Stable identifiers [uaxid.def.stable]

An implementation of UAX #31 may choose to guarantee that identifiers are stable across versions of the Unicode Standard.
Once a string qualifies as an identifier it does so in all future versions.
C++ does not make this guarantee, except to the extent that UAX #31 guarantees the stability of the XID_Start and XID_Continue properties.