I may lack sufficient context for this to be a useful PR, so please feel free to close if this is not welcome.

For no particular reason I saw an analogous PR to another R package repo in my GitHub home page:

marzer/tomlplusplus#263

I searched the main R package repo for other similarly-affected packages, which includes {clustermq}, which bundles this repo as a submodule:

https://github.com/mschubert/clustermq/tree/master/src

Here's the documentation I could find about why this matters:

https://en.cppreference.com/w/cpp/language/user_literal

// error: all names that begin with underscore followed by uppercase
// letter are reserved (NOTE: a space between "" and _).
double operator"" _Z(long double);