The missing harm of manual dispatch in Julia(andreaskroepelin.de)

    function foo(x)
      # some lines of code
      y = x isa Int ? 2.0 : 3x
      # some other lines that do things with y
    end

    foo_magic(x::Int) = 2.0
    foo_magic(x) = 3x

    function foo(x)
      # some lines of code
      y = foo_stuff(x)
      # some other lines that do things with y
    end

  julia> a = 20
  20

  julia> foo(x::(a < 10 ? Int : Float64)) = "thing"
  foo (generic function with 1 method)

  julia> methods(foo)
  # 1 method for generic function "foo":
  [1] foo(x::Float64) in Main at REPL[2]:1

    foo(x::Int) = x + 1
    foo(x::Float64) = x + 2
    foo(x::String) = "$(x) $(x)"

    function eval_user_input()
        user_input = readline(stdin)
        user_obj = eval(Meta.parse(user_input))
        return foo(user_obj)
    end

    julia> @code_warntype eval_user_input()
    Variables
      #self#::Core.Compiler.Const(eval_user_input, false)
      user_input::String
      user_obj::Any

    Body::Union{Float64, Int64, String}
    1 ─      (user_input = Main.readline(Main.stdin))
    │   %2 = Base.Meta.parse::Core.Compiler.Const(Base.Meta.parse, false)
    │   %3 = (%2)(user_input)::Any
    │        (user_obj = Main.eval(%3))
    │   %5 = Main.foo(user_obj)::Union{Float64, Int64, String}
    └──      return %5

    10 ┄ %20 = φ (#8 => %12, #9 => %18)::String
    │    %21 = Base.Meta.parse::Core.Compiler.Const(Base.Meta.parse, false)
    │    %22 = invoke Base.Meta.:(var"#parse#4")(true::Bool, true::Bool, %21::typeof(Base.Meta.parse), %20::String)::Any
    │    %23 = Main.eval(%22)::Any
    │    %24 = (isa)(%23, String)::Bool
    └───       goto #12 if not %24
    11 ─ %26 = π (%23, String)
    │    %27 = invoke Base.string(%26::String, " "::String, %26::Vararg{String,N} where N)::String
    └───       goto #17
    12 ─ %29 = (isa)(%23, Float64)::Bool
    └───       goto #14 if not %29
    13 ─ %31 = π (%23, Float64)
    │    %32 = Base.sitofp(Float64, 2)::Float64
    │    %33 = Base.add_float(%31, %32)::Float64
    └───       goto #17
    14 ─ %35 = (isa)(%23, Int64)::Bool
    └───       goto #16 if not %35
    15 ─ %37 = π (%23, Int64)
    │    %38 = Base.add_int(%37, 1)::Int64
    └───       goto #17