Getting parent's parametric types parameter

General Usage
2

You mean like this?

struct SomeFoo{T<:Integer} <: AbstractFoo{T}
  field1::T 
  field2::Vector{T}
end

julia> SomeFoo(1, [1, 2])
SomeFoo{Int64}(1, [1, 2])

I should also ask why you need the limitation at all? This would stop people using integers or floats with Unitful units, or similar things…