Annex G (informative) Ill-formed, no diagnostic required [ifndr]

G.3 [basic]: Basics [ifndr.basic]

G.3.6[ifndr:basic.def.odr.unnamed.enum.same.type]
Specified in: [basic.def.odr]

1
#
Having multiple unnamed enumeration definitions in the same scope that have the same first enumerator name and do not have typedef names for linkage purposes ([dcl.enum]) that are not the same enumeration is ill-formed, no diagnostic required.
2
#
[Example 1: 

Source file "a.h":enum { a };

Source file "b.h":enum { a, b };

Source file "main.cpp":import "a.h"; import "b.h"; auto n = decltype(a)::b; // IFNDR, more than one unnamed enum definition reachable at // this point but their types are not the same — end example]