Currently the phrase to grant this permission is:

The template parameter T of LibraryTemplate may be an incomplete type.

Two problems:

1. The timing is unclear. We always allow specializations like LibraryTemplate<Incomp>* p;

2. To the users of a template, the correct terminology should be "argument" rather than "parameter".

Suggested resolution:

In an instantiation of LibraryTemplate, an incomplete type may be used as the template argument for the template parameter T.

as shown here.

Or, to copy N4510's wording:

An incomplete type T may be used when instantiating LibraryTemplate.