There's no hidden dirty magic, but you're right: Complex numbers require `Real` components and Rationals require `Integer` numerators and denominators. Both `Real` and `Integer` are simply abstract types in the numeric type tree, but you're free to make your own. You can see how this works directly in their docstrings — it's that type parameter in curly braces that defines it, and `<:` means subtype:

    help?> Complex
    search: Complex complex ComplexF64 ComplexF32 ComplexF16 completecases
    
      Complex{T<:Real} <: Number
    
      Complex number type with real and imaginary part of type T.