If this is all you want, you should copy the example of Float16 instead: julia/boot.jl at 66c9f6a9b58130d35fa9cf10a7e16daf5847e575 · JuliaLang/julia · GitHub
But it doesn’t seem like it’s true that you want only this: it sounds like you want “all existing methods for Float64, except for the constructor”, which is kind of the mirror image of creating a new primitive type.