Why can I `deepcopy(nothing)` but not `copy(nothing)`?

[stevengj]

See previous discussion of copy for immutable objects:

• Coordinating community efforts to enrich function docs - #8 by johnmyleswhite
• copy not implemented for String · Issue #31995 · JuliaLang/julia · GitHub
• Define copy for immutable values · Issue #45143 · JuliaLang/julia · GitHub