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

[jeff.benshetler at stackpath dot com]

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

--- Comment #1 from Jeff Benshetler <jeff.benshetler at stackpath dot com> ---
Created attachment 41747
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=41747&action=edit
Source test case that triggers the bug; corresponds to .ii file

Added in an abundance of caution. 
No header files are used. 
The first constructor has [[maybe_used]] on the first constructor parameter and
it triggers the bug.
The second constructor uses [[maybe_unused]] on the second constructor
parameter and it is fine.