[Bug libstdc++/89562] std::filesystem::copy truncates files before byte with value 0x1A

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89562

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
std::filesystem isn't expected to work on Windows until gcc 9:
https://gcc.gnu.org/gcc-9/changes.html#libstdcxx