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

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

G.3.2[ifndr:basic.def.odr.minimum.one.def]
Specified in: [basic.def.odr]

1
#
Not having a definition for a function or variable that is odr-used from a non-discarded statement ([stmt.if]) is ill-formed, no diagnostic required.
2
#
[Example 1: auto f() { struct A {}; return A{}; } decltype(f()) g(); auto x = g(); // IFNDR, function g is used but not defined in this translation unit, and cannot // be defined in any other translation unit because its type does not have linkage — end example]