Accessing field N of Vararg{T} is an undefined access on Julia 1.7

[darsnack]

I ran into this trying to fix BSON.jl: JuliaIO/BSON.jl#102.

julia> versioninfo()
Julia Version 1.6.0
Commit f9720dc2eb (2021-03-24 12:55 UTC)
Platform Info:
  OS: macOS (x86_64-apple-darwin19.6.0)
  CPU: Intel(R) Core(TM) i7-6700HQ CPU @ 2.60GHz
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-11.0.1 (ORCJIT, skylake)
Environment:
  JULIA_NUM_THREADS = 4

julia> x = Vararg{Any}
Vararg{Any, N} where N

julia> fieldnames(typeof(x))
(:var, :body)

julia> x.var
N

julia> x.body
Vararg{Any, N}

julia> versioninfo()
Julia Version 1.7.0-beta3.0
Commit e76c9dad42 (2021-07-07 08:12 UTC)
Platform Info:
  OS: macOS (x86_64-apple-darwin18.7.0)
  CPU: Intel(R) Core(TM) i7-6700HQ CPU @ 2.60GHz
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-12.0.0 (ORCJIT, skylake)
Environment:
  JULIA_NUM_THREADS = 4

julia> x = Vararg{Any}
Vararg{Any}

julia> fieldnames(typeof(x))
(:T, :N)

julia> x.T
Any

julia> x.N
ERROR: UndefRefError: access to undefined reference
Stacktrace:
 [1] getproperty(x::Core.TypeofVararg, f::Symbol)
   @ Base ./Base.jl:42
 [2] top-level scope
   @ REPL[48]:1

I'm not sure what the correct fix here should be w.r.t. BSON.jl, but it seems like accessing x.N should not error or fieldnames(typeof(x)) should not contain :N.

[JeffBezanson]

The N field exists but is undefined when the length is unspecified. In general objects can have undefined fields, so that needs to be handled by any code that looks at object fields generically. For example

julia> struct X
       field
       X() = new()
       end

julia> X().field
ERROR: UndefRefError: access to undefined reference

julia> fieldnames(X)
(:field,)