CWG2848 [temp.explicit] Allowing omitting empty template argument list for default arguments during explicit instantiation #489
Description
Full name of submitter: Anoop Rana
Reference (section label): [temp.explicit]
Issue description: Currently the wording in [temp.explicit] does not allow for empty <> to be omitted when all of the template arguments can be obtained from the default arguments. Additionally, it also does not allow for the trailing template argument to be left unspecified if it can be obtained from default argument. The below example shows the issue:
template<class T> class Array { /* ... */ };
template<class T, typename U = double> void sort(Array<T>& v) { /* ... */ }
template void sort<>(Array<int>&); //ill-formed as per current wording
template void sort(Array<double>&); //ill-formed as per current wording
The current wording in temp.explicit#8 states:
A trailing template-argument can be left unspecified in an explicit instantiation of a function template specialization or of a member function template specialization provided it can be deduced ([temp.deduct.decl]).
If all template arguments can be deduced, the empty template argument list <> may be omitted.
Suggested resolution:
Change [temp.explicit#8] to as indicated/highlighted below:
A trailing template-argument can be left unspecified in an explicit instantiation of a function template specialization or of a member function template specialization provided it can be deduced ([temp.deduct.decl]). or obtained from default template-arguments. If all template arguments can be deduced or obtained from default template-arguments, the empty template argument list <> may be omitted.
After the suggested changes, the two statements(involving explicit instantiation) will be well-formed.