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

G.6 [dcl]: Declarations [ifndr.dcl]

G.6.6[ifndr:dcl.link.mismatched.language.linkage]
Specified in: [dcl.link]

1
#
If two declarations of an entity do not have the same language linkage and neither is reachable from the other the program is ill-formed, no diagnostic required.
2
#
[Example 1: 

Translation unit #1:extern "C" { void f(); }

Translation unit #2:extern "C++" { void f(); } // IFNDR, different language linkage — end example]