https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92505
--- Comment #2 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
cxx_eval_component_reference has:
2967 if (DECL_MUTABLE_P (part))
2968 {
2969 if (!ctx->quiet)
2970 error ("mutable %qD is not usable in a constant expression",
part);
2971 *non_constant_p = true;
2972 }
Added in https://gcc.gnu.org/ml/gcc-patches/2011-10/msg02593.html
I will need to do a bit of archeology on the isocpp mailing list to see the
discussions.
