There isn’t such a type for floating-point numbers; you could define one, of course, but this is something that’s probably better done with a runtime argument check:
function f(x::Real)
x ≥ 0 || throw(DomainError("negative x=$x not allowed"))
...
end
Note that if x is an Unsigned type (like UInt, and also things like Rational{UInt}), the compiler will know that x ≥ 0 is always true and optimize out the check.