This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 110b. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.
[Accepted at the November, 2020 meeting as part of paper P1787R6 and moved to DR at the February, 2021 meeting.]
According to 188.8.131.52 [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-seqopt 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 templateopt simple-template-id ;
This implies that class template partial specializations cannot be forward-declared, which is probably unintentional.
Notes from the November, 2016 meeting:
CWG felt that forward declarations of partial specializations should be allowed.