The following program will not compile:
int main(string[] args)
{
	int _tmp0_, _tmp1_;
	
	_tmp0_ = -5;
	_tmp1_ = -(_tmp0_);
	
	stdout.printf("|%d| = %d\n", _tmp0_, _tmp1_);
	
	return 0;
}

Errors:
$ valac main.vala
/tmp/main.vala.c: In function ‘_vala_main’:
/tmp/main.vala.c:21:8: error: conflicting types for ‘_tmp0_’
  FILE* _tmp0_;
        ^
/tmp/main.vala.c:19:7: note: previous definition of ‘_tmp0_’ was here
  gint _tmp0_ = 0;
       ^
/tmp/main.vala.c:22:9: warning: assignment makes pointer from integer without a cast [enabled by default]
  _tmp0_ = -5;
         ^
/tmp/main.vala.c:23:11: error: wrong type argument to unary minus
  _tmp1_ = -_tmp0_;
           ^
/tmp/main.vala.c: In function ‘main’:
/tmp/main.vala.c:32:2: warning: ‘g_type_init’ is deprecated (declared at /usr/include/glib-2.0/gobject/gtype.h:669) [-Wdeprecated-declarations]
  g_type_init ();
  ^
error: cc exited with status 256
Compilation failed: 1 error(s), 0 warning(s)

Should at least be noted that the documentation ...