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

E.2 R1 Default identifiers [uaxid.def]

E.2.1 General [uaxid.def.general]

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UAX #31 specifies a default syntax for identifiers based on properties from the Unicode Character Database, UAX #44.
The general syntax is 
<Identifier> := <Start> <Continue>* (<Medial> <Continue>+)*
where <Start> has the XID_Start property, <Continue> has the XID_Continue property, and <Medial> is a list of characters permitted between continue characters.
For C++ we add the character U+005f low line, or _, to the set of permitted <Start> characters, the <Medial> set is empty, and the <Continue> characters are unmodified.
In the grammar used in UAX #31, this is 
<Identifier> := <Start> <Continue>*
<Start> := XID_Start + U+005f
<Continue> := <Start> + XID_Continue
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This is described in the C++ grammar in [lex.name], where identifier is formed from identifier-start or identifier followed by identifier-continue.

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

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If an implementation of UAX #31 wishes to allow format characters such as U+200d zero width joiner or U+200c zero width non-joiner it must define a profile allowing them, or describe precisely which combinations are permitted.
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C++ does not allow format characters in identifiers, so this does not apply.

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

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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.
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C++ does not make this guarantee, except to the extent that UAX #31 guarantees the stability of the XID_Start and XID_Continue properties.