[Bug libstdc++/61946] [4.8/4.9/4.10 Regression] rope construction, passing allocator referenct without const

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61946

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Fixed on trunk so far