Help newbie to understand type stability please

General Usage
2

You make x change its type. Did you mean x .*= 1? Or you can use another variable, as in

function good3(x::Array{T,1}) where T<:Float64
    n = length(x)
    y = x * 1
    [y[i] for i in 1:n]
end

Also see this suggestion in the manual.