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

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

G.6.2[ifndr:dcl.inline.missing.on.definition]
Specified in: [dcl.inline]

1
#
If a function or variable with external or module linkage is declared inline but there is no inline declaration reachable from the end of some definition domain the program is ill-formed, no diagnostic required.
2
#
[Example 1: 

Translation unit #1:inline int f();

Translation unit #2:int f() { return 17; } // IFNDR, end of definition domain but no inline declaration of f is reachable — end example]