julia> using Pkg, Revise

julia> Pkg.generate("DemonstrationPackage")
  Generating  project DemonstrationPackage:
    DemonstrationPackage/Project.toml
    DemonstrationPackage/src/DemonstrationPackage.jl
Dict{String, Base.UUID} with 1 entry:
  "DemonstrationPackage" => UUID("59767476-78e9-4613-8abe-91176a478dbc")

(@v1.11) pkg> activate DemonstrationPackage/
  Activating project at `~/.julia/dev/DemonstrationPackage`

julia> open("DemonstrationPackage/src/DemonstrationPackage.jl", "w") do io
           print(io, """
           module DemonstrationPackage
           
           struct NestedDict{K, V} <: AbstractDict{K, Union{V, NestedDict}} end

           end
           """)
       end

julia> using DemonstrationPackage
[ Info: Precompiling DemonstrationPackage [59767476-78e9-4613-8abe-91176a478dbc] 

julia> open("DemonstrationPackage/src/DemonstrationPackage.jl", "a") do io
           println(io, "# Comment")
       end

julia> 1+1
┌ Error: Failed to revise /home/x/.julia/dev/DemonstrationPackage/src/DemonstrationPackage.jl
│   exception =
│    invalid redefinition of constant DemonstrationPackage.NestedDict
│    Stacktrace:
│     [1] top-level scope
│       @ ~/.julia/dev/DemonstrationPackage/src/DemonstrationPackage.jl:3
│    Revise evaluation error at /home/x/.julia/dev/DemonstrationPackage/src/DemonstrationPackage.jl:3
│    
└ @ Revise ~/.julia/packages/Revise/CawM1/src/packagedef.jl:776
┌ Warning: The running code does not match the saved version for the following files:
│ 
│   /home/x/.julia/dev/DemonstrationPackage/src/DemonstrationPackage.jl
│ 
│ If the error was due to evaluation order, it can sometimes be resolved by calling `Revise.retry()`.
│ Use Revise.errors() to report errors again. Only the first error in each file is shown.
│ Your prompt color may be yellow until the errors are resolved.
└ @ Revise ~/.julia/packages/Revise/CawM1/src/packagedef.jl:893
2

shell> cat DemonstrationPackage/src/DemonstrationPackage.jl
module DemonstrationPackage

struct NestedDict{K, V} <: AbstractDict{K, Union{V, NestedDict}} end

end
# Comment