[Bug libstdc++/108952] Regression in uses_allocator_construction_args for pair of rvalue references

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108952

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Peter Dimov from comment #4)
> An easy fix is to use `std::get<0>` instead of `.first`.

Yeah I tried that, but I think there's an additional change needed (or I messed
something up).