https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69632
--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> --- GCC 9+ produce: <source>:1:28: error: 'parameter' function with trailing return type not declared with 'auto' type specifier 1 | int foo (long (int) -> char); | ^~~~