[Bug c++/63356] Compilation failure where clang does not have problems

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63356

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
You should have read https://gcc.gnu.org/bugs/ and attached preprocessed code
anyway, not everyone has Boost 1.56 already installed.