According to people on #llvm, this is indeed not allowed by c++ standard:
[01:33] <coppro> what good would mutable do on a reference?
[01:33] <dgregor> orzel: gcc is wrong to allow "mutable" on references
[01:33] <coppro> just remove mutable; it won't damage the code at all
[01:34] <dgregor> "The mutable specifier can be applied only to names of
class data members (9.2) and cannot be applied to
[01:34] <dgregor> names declared const or static, and cannot be applied to
reference members."
[01:34] <coppro> constness is not passed from an object to the referents of
its members anyways
17d080edf2