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

G.10 [temp]: Templates [ifndr.temp]

G.10.1[ifndr:temp.pre.reach.def]
Specified in: [temp.pre]

1
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A definition of a function template, member function of a class template, variable template, or static data member of a class template that is not reachable from the end of every definition domain ([basic.def.odr]) in which it is implicitly instantiated ([temp.inst]) and whose corresponding specialization is not explicitly instantiated ([temp.explicit]) in some translation unit is ill-formed, no diagnostic required.
2
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[Example 1: 

Source file "a.h":template <typename T> void f();

Source file "a.cpp":#include "a.h" int main() { f<int>(); // IFNDR, function template implicitly instantiated but not reachable definition } — end example]