templates - Maybe types in D -

i'm trying define value type either holds size_t or null (which mean 'maybe type'). want able (where relevant type maybe!size_t:

 maybe!size_t something_which_could_fail (int foo) {       if (foo < 0) { return null;}       else { return foo;}  } 

how implement such thing? ideally, able make template (so have other maybe types well), , have conversions maybe non-maybe types possible above (although don't mind typecasting if not possible). nice if this:

maybe!size_t = 50; maybe!size_t b = null; 

what nullable type in phobos library? http://dlang.org/phobos/std_typecons.html#.nullable

and other info in d forum: d forum thread option(maybe) type