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

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

G.10.9[ifndr:temp.point.diff.pt.diff.meaning]
Specified in: [temp.point]

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A specialization for a class template has at most one point of instantiation within a translation unit.
A specialization for any template may have points of instantiation in multiple translation units.
If two different points of instantiation give a template specialization different meanings according to the one-definition rule ([basic.def.odr]), the program is ill-formed, no diagnostic required.
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[Example 1: 

Source file "a.h":#include <type_traits> template <typename T, typename Enabler = void> struct is_complete : std::false_type {}; template <typename T> struct is_complete<T, std::void_t<decltype(sizeof(T) != 0)>> : std::true_type {};

Source file "a.cpp":#include "a.h" struct X; static_assert(!is_complete<X>::value); // IFNDR, different specialization selected in "b.cpp"

Source file "b.cpp":#include "a.h" struct X { }; static_assert(is_complete<X>::value); // IFNDR, different specialization selected in "a.cpp" — end example]