Initialise array without specific types

General Usage Performance
16

You need to pass an instance (or, example) of a type, so in this case any real number would work like so

function do_something_3(x::Int, y::T) where {T<:Real} 

  
    intermediate_thing = Array{T}(undef,(3,4))

    intermediate_thing[1,1] = 3.0
    
   return(intermediate_thing[1,1] * x)
end

julia> do_something_3(1, 1.2)
3.0

Also, note you’re making another mistake when you write do_something(1.2, Float16). The argument x in do_something(x, y) needs to be an integer and not a float. For example, this fails (notice the stacktrace)

julia> do_something_3(1.0, 1.2)

ERROR: MethodError: no method matching do_something_3(::Float64, ::Float64)

Closest candidates are:
  do_something_3(::Int64, ::T) where T<:Real at REPL[293]:1

but this doesn’t

julia> do_something_3(1, 1.2)
3.0