Notes from the November, 2016 meeting:

CWG felt that forward declarations of partial specializations should be allowed.

[Accepted at the November, 2020 meeting as part of paper P1787R6 and moved to DR at the February, 2021 meeting.]

According to 9.2.9.5 [dcl.type.elab] paragraph 1,

If an elaborated-type-specifier is the sole constituent of a declaration, the declaration is ill-formed unless it is an explicit specialization (13.9.4 [temp.expl.spec]), an explicit instantiation (13.9.3 [temp.explicit]) or it has one of the following forms:

class-key attribute-specifier-seq_opt identifier ;
friend class-key ::_opt identifier ;
friend class-key ::_opt simple-template-id ;
friend class-key nested-name-specifier identifier ;
friend class-key nested-name-specifier template_opt simple-template-id ;

This implies that class template partial specializations cannot be forward-declared, which is probably unintentional.