This segfaults currently because the call to method_int8_in tries to invoke the corresponding vfunc slot in the class struct for NoImplementation. But that's NULL since NoImplementation doesn't provide an implementation.

{ package NoImplementation; use Glib::Object::Subclass 'GI::Object'; }

{ my $foo = NoImplementation->new; local $@; eval { $foo->method_int8_in (23) }; like ($@, qr/method_int8_in/); }