round(x::Quantity) = round(typeof(x), x)

[jariji]

julia> round(1.2u"s")
ERROR: specify the type of the quantity to convert to when rounding quantities. Example: round(typeof(1u"m"), 137u"cm").
Stacktrace:
     ⋮ internal @ Base, Unitful
 [3] round(x::Quantity{Float64, 𝐓, Unitful.FreeUnits{(s,), 𝐓, nothing}}, r::RoundingMode{:Nearest}; kwargs::@Kwargs{})
   @ Unitful ~/.julia/packages/Unitful/6r8Hq/src/quantities.jl:327
Use `err` to retrieve the full stack trace.

julia> let x = 1.2u"s"; round(typeof(x), x; digits=2) end
1.2 s

What about defining round(x::Quantity; kw...) = round(typeof(x), x; kw...) so I don't have to pass it myself? Would that work?

[sostock]

It would “work”, but then rounding wouldn’t be an operation that is invariant under unit conversion anymore. That’s why it’s forbidden. Consider

julia> round(x::Quantity; kw...) = round(typeof(x), x; kw...)
round (generic function with 89 methods)

julia> 1.2m == 120cm
true

julia> round(1.2m) == round(120cm)
false

Previous discussion: #326, #328