[Bug c++/81429] maybe_unused attribute triggers syntax error when used on first argument to a constructor

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81429

--- Comment #9 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Fixed on trunk, will backport to 9.3 later.