Packages outside sysimage without versions in Project file are not compatible with Pkg

[vtjnash]

This reproducibly fails for me, when Pkg is not baked into the system image:

julia> Pkg.update()
    Updating registry at `~/.julia/registries/General`
    Updating git-repo `https://github.com/JuliaRegistries/General.git`
    Updating git-repo `https://github.com/chriselrod/VectorizationBase.jl.git`
No Changes to `~/.julia/environments/v1.1/Project.toml`
No Changes to `~/.julia/environments/v1.1/Manifest.toml`
ERROR: MethodError: Cannot `convert` an object of type 
  Nothing to an object of type 
  Union{Pkg.Types.UpgradeLevel, VersionNumber, Pkg.Types.VersionSpec}
Closest candidates are:
  convert(::Type{T}, ::T) where T at essentials.jl:205
Stacktrace:
  [1] Pkg.Types.PackageSpec(name::String, uuid::Base.UUID, version::Nothing, tree_hash::Nothing, repo::Pkg.Types.GitRepo, path::Nothing, pinned::Bool, mode::Pkg.Types.PackageMode)
    @ Pkg.Types /data/vtjnash/julia/usr/share/julia/stdlib/v1.6/Pkg/src/Types.jl:91
  [2] Pkg.Types.PackageSpec(; name::String, uuid::Base.UUID, version::Nothing, tree_hash::Nothing, repo::Pkg.Types.GitRepo, path::Nothing, pinned::Bool, mode::Pkg.Types.PackageMode)
    @ Pkg.Types ./util.jl:444
  [3] precompile(ctx::Pkg.Types.Context; internal_call::Bool, io::Base.TTY)
    @ Pkg.API /data/vtjnash/julia/usr/share/julia/stdlib/v1.6/Pkg/src/API.jl:959
  [4] _auto_precompile(ctx::Pkg.Types.Context; kwargs::Base.Iterators.Pairs{Union{}, Union{}, Tuple{}, NamedTuple{(), Tuple{}}})
    @ Pkg.API /data/vtjnash/julia/usr/share/julia/stdlib/v1.6/Pkg/src/API.jl:912
  [5] _auto_precompile
    @ /data/vtjnash/julia/usr/share/julia/stdlib/v1.6/Pkg/src/API.jl:911 [inlined]
  [6] #up#38
    @ /data/vtjnash/julia/usr/share/julia/stdlib/v1.6/Pkg/src/API.jl:72 [inlined]
  [7] up
    @ /data/vtjnash/julia/usr/share/julia/stdlib/v1.6/Pkg/src/API.jl:70 [inlined]
  [8] up(; name::Nothing, uuid::Nothing, version::Nothing, url::Nothing, rev::Nothing, path::Nothing, mode::Pkg.Types.PackageMode, subdir::Nothing, kwargs::Base.Iterators.Pairs{Union{}, Union{}, Tuple{}, NamedTuple{(), Tuple{}}})
    @ Pkg.API /data/vtjnash/julia/usr/share/julia/stdlib/v1.6/Pkg/src/API.jl:87
  [9] up()
    @ Pkg.API /data/vtjnash/julia/usr/share/julia/stdlib/v1.6/Pkg/src/API.jl:79
 [10] top-level scope
    @ none:1

Failing on:

pkgid = ArgTools [0dad84c5-d112-42e6-8d28-ef12dabb789f]
pkgent = PackageEntry(
  name = ArgTools
)

[IanButterworth]

This is breaking because of an assumption that the only packages that will be missing the version field in the manifest will be in the sysimage (i.e. stdlibs), which we skip entirely during precomp.

Could we expand the union type of PackageSpec.version to include nothing, like the other fields?

Otherwise a better fix may be to disable the precomp suspension mechanism for packages like this that aren't in the sysimage. That's probably a good idea given the lack of version would mean we couldn't tell when the package has changed.

[KristofferC]

Until we have a well-defined description of what an stdlib actually is, it is hard to write working code for it. Right now, they have been defined in Pkg "by inspection", i.e. how they have actually been used in Base. If they uses in Base changes, those assumptions are likely invalid.