Correct way of: f(::T) where T = zero(T)?

What is the correct way to pass only a type to control dispatch and use that type inside the function? As the title says, I want to do something like:

 f(::T) where T = zero(T)

Is this one?

julia> f(T::DataType) = zero(T)
f (generic function with 2 methods)

julia> f(Float64)

edit: that does not work because the type of T will be DataType, thus the function will not specialize for T.

f(::Type{T}) where {T} = zero(T) 

Similar question two weeks ago: Dispatch on type only

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