According to 9.2.2 [dcl.stc] paragraph 9,

The mutable specifier can be applied only to names of class data members (11.4 [class.mem]) and cannot be applied to names declared const or static, and cannot be applied to reference members.

This is similar to issue 1686 in that the restriction appears to apply only to declarations in which the const keyword appears directly. It should instead apply to members with const-qualified types, regardless of how the qualification was achieved.