Core Working Group defect report 993 was created to address this issue:
993. Freedom to perform instantiation at the end of the translation unit
Section: 14.6.4.1 [temp.point] Status: C++11 Submitter: John Spicer Date: 6 March, 2009
[Voted into the WP at the March, 2011 meeting.]The intent is that it is a permissible implementation technique to do template instantiation at the end of a translation unit rather than at an actual point of instantiation. This idea is not reflected in the current rules, however.
Proposed resolution (January, 2011):
Change 14.6.4.1 [temp.point] paragraph 7 as follows:
A specialization for a function template, a member function template, or of a member function or static data member of a class template may have multiple points of instantiations within a translation unit, and in addition to the points of instantiation described above, for any such specialization that has a point of instantiation within the translation unit, the end of the translation unit is also considered a point of instantiation. A specialization for a class template…
Paragraph 14.6.4.1/7 in C++11 is 14.6.4.1/8 in N3936:
A specialization for a function template, a member function template, or of a member function or static
data member of a class template may have multiple points of instantiations within a translation unit, and
in addition to the points of instantiation described above, for any such specialization that has a point
of instantiation within the translation unit, the end of the translation unit is also considered a point of
instantiation. 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 (3.2), the program is ill-formed, no diagnostic required.
So yes, it is allowable for implementations to delay the point of instantiation of templates to the end of the translation unit.