According to 126.96.36.199 [expr.prim.id.unqual] paragraph 1,
An identifier is an id-expression provided it has been
suitably declared (Clause 7 [dcl.dcl]).
Not only is an identifier an id-expression by
(grammatical) definition, declarator-id is defined in terms of
id-expression, which makes this circular. If the intention was
to disallow use of undeclared identifiers as primary expressions, this
should be altered accordingly.