no method matching joinpath(::Nothing) during download_source on 1.6.2-pre

[IanButterworth]

Happening here https://github.com/IanButterworth/SystemBenchmark.jl/pull/48/checks?check_run_id=2994910850#step:5:6

on 1.6-nightly on all platforms

Julia Version 1.6.2-pre.59
Commit 134c34397c (2021-07-01 10:27 UTC)
Platform Info:
  OS: macOS (x86_64-apple-darwin19.5.0)
  CPU: Intel(R) Xeon(R) CPU E5-1650 v2 @ 3.50GHz
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-11.0.1 (ORCJIT, ivybridge)

[ Info: Attempting to clone the General registry
     Cloning registry from "https://github.com/JuliaRegistries/General.git"
       Added registry `General` to `~/.julia/registries/General`
[ Info: Successfully added the General registry
ERROR: MethodError: no method matching joinpath(::Nothing)
Closest candidates are:
  joinpath(::AbstractString) at path.jl:251
  joinpath(::AbstractString, ::AbstractString...) at path.jl:296
Stacktrace:
  [1] stat(path::Nothing)
    @ Base.Filesystem ./stat.jl:109
  [2] ispath(path::Nothing)
    @ Base.Filesystem ./stat.jl:311
  [3] download_source(ctx::Pkg.Types.Context, pkgs::Vector{Pkg.Types.PackageSpec}, urls::Dict{Base.UUID, Vector{String}}; readonly::Bool)
    @ Pkg.Operations /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.6/Pkg/src/Operations.jl:743
  [4] #download_source#57
    @ /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.6/Pkg/src/Operations.jl:733 [inlined]
  [5] download_source
    @ /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.6/Pkg/src/Operations.jl:731 [inlined]
  [6] instantiate(ctx::Pkg.Types.Context; manifest::Nothing, update_registry::Bool, verbose::Bool, platform::Base.BinaryPlatforms.Platform, allow_build::Bool, allow_autoprecomp::Bool, kwargs::Base.Iterators.Pairs{Union{}, Union{}, Tuple{}, NamedTuple{(), Tuple{}}})
    @ Pkg.API /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.6/Pkg/src/API.jl:1406
  [7] build(ctx::Pkg.Types.Context, pkgs::Vector{Pkg.Types.PackageSpec}, verbose::Bool)
    @ Pkg.Operations /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.6/Pkg/src/Operations.jl:906
  [8] build(ctx::Pkg.Types.Context, pkgs::Vector{Pkg.Types.PackageSpec}; verbose::Bool, kwargs::Base.Iterators.Pairs{Symbol, IOContext{Base.PipeEndpoint}, Tuple{Symbol}, NamedTuple{(:io,), Tuple{IOContext{Base.PipeEndpoint}}}})
    @ Pkg.API /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.6/Pkg/src/API.jl:900
  [9] build(pkgs::Vector{Pkg.Types.PackageSpec}; io::IOContext{Base.PipeEndpoint}, kwargs::Base.Iterators.Pairs{Symbol, Bool, Tuple{Symbol}, NamedTuple{(:verbose,), Tuple{Bool}}})
    @ Pkg.API /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.6/Pkg/src/API.jl:80
 [10] build(; name::Nothing, uuid::Nothing, version::Nothing, url::Nothing, rev::Nothing, path::Nothing, mode::Pkg.Types.PackageMode, subdir::Nothing, kwargs::Base.Iterators.Pairs{Symbol, Bool, Tuple{Symbol}, NamedTuple{(:verbose,), Tuple{Bool}}})
    @ Pkg.API /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.6/Pkg/src/API.jl:96
 [11] top-level scope
    @ none:1
Error: Process completed with exit code 1.

[IanButterworth]

This appears to be another way that a manifest from another julia version can cause errors

In 1.6.2:

Manifest resolved in 1.6.2

No error

Manifest resolved in 1.7

ERROR: TypeError: in typeassert, expected VersionNumber, got a value of type Pkg.Types.VersionSpec
Stacktrace:
 [1] load_urls(ctx::Pkg.Types.Context, pkgs::Vector{Pkg.Types.PackageSpec})
   @ Pkg.Operations /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.6/Pkg/src/Operations.jl:545
 [2] #download_source#57
   @ /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.6/Pkg/src/Operations.jl:732 [inlined]
 [3] download_source
   @ /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.6/Pkg/src/Operations.jl:731 [inlined]

Manifest resolved in 1.8

ERROR: MethodError: no method matching joinpath(::Nothing)
Closest candidates are:
  joinpath(::AbstractString) at path.jl:251
  joinpath(::AbstractString, ::AbstractString...) at path.jl:296
Stacktrace:
  [1] stat(path::Nothing)
    @ Base.Filesystem ./stat.jl:109
  [2] ispath(path::Nothing)
    @ Base.Filesystem ./stat.jl:311
  [3] download_source(ctx::Pkg.Types.Context, pkgs::Vector{Pkg.Types.PackageSpec}, urls::Dict{Base.UUID, Vector{String}}; readonly::Bool)
    @ Pkg.Operations /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.6/Pkg/src/Operations.jl:743

[IanButterworth]

IIUC the backport of #2620 in #2628 omitted backporting the manifest julia_version incompatibility warning to 1.6

If we don't backport the warning, I think there may be various odd errors like above that don't have an actionable warning

[IanButterworth]

I propose #2666 goes into 1.6.2