https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91215
--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> --- Also read https://gcc.gnu.org/gcc-8/porting_to.html : -Wreturn-type is enabled by default G++ now assumes that control never reaches the end of a non-void function (i.e. without reaching a return statement). This means that you should always pay attention to -Wreturn-type warnings, as they indicate code that can misbehave when optimized. To tell the compiler that control can never reach the end of a function (e.g. because all callers enforce its preconditions) you can suppress -Wreturn-type warnings by adding __builtin_unreachable: char signchar(int i) // precondition: i != 0 { if (i > 0) return '+'; else if (i < 0) return '-'; __builtin_unreachable(); } Because -Wreturn-type is now enabled by default, G++ will warn if main is declared with an implicit int return type (which is non-standard but allowed by GCC). To avoid the warning simply add a return type to main, which makes the code more portable anyway.
