[Moved to DR at the November, 2014 meeting.]

Proposed resolution (February, 2014):

Change 13.8 [temp.res] paragraph 8 as follows:

...If a type used in a non-dependent name is incomplete at the point at which a template is defined but is complete at the point at which an instantiation is done, and if the completeness of that type affects whether or not the program is well-formed or affects the semantics of the program, the program is ill-formed; no diagnostic is required.

[Note: If a template is instantiated...

Various characteristics of entities referred to by a non-dependent reference in a template can change between the definition context and the point of instantiation of a specialization of that template. These include initialization (which affects whether an object can be used in a constant expression), function and template default arguments, and the completeness of types. There is implementation divergence as to whether these are checked in the definition context or at the point of instantiation. Presumably a rule is needed to make it ill-formed, no diagnostic required, if the validity of such a reference changes between the two contexts.