Proposed resolution (October, 2015):

1. Change the grammar in 9.7.1 [dcl.enum] paragraph 1 as follows:

opaque-enum-declaration:
enum-key attribute-specifier-seq_opt nested-name-specifier_opt identifier enum-base_opt ;

2. Add the following at the end of 9.7.1 [dcl.enum] paragraph 1:

If an opaque-enum-declaration contains a nested-name-specifier, the declaration shall be an explicit specialization (13.9.4 [temp.expl.spec]).

[Adopted at the February, 2016 meeting.]

There is no syntax currently for declaring an explicit specialization of a member scoped enumeration. A declaration (not a definition) of such an explicit specialization most resembles an opaque-enum-declaration, but the grammar for that requires that the name be a simple identifier, which will not be the case for an explicit specialization of a member enumeration. This could be remedied by adding a nested-name-specifier to the grammar with a restriction that a nested-name-specifier only appear in an explicit specialization.