julia> using Pkg

julia> Pkg.add("Cxx")
  Updating registry at `C:\Users\andre\.julia\registries\General`
  Updating git-repo `https://github.com/JuliaRegistries/General.git`
 Resolving package versions...
ERROR: Unsatisfiable requirements detected for package Cxx [a0b5b9ef]:
 Cxx [a0b5b9ef] log:
 ├─possible versions are: [0.0.1-0.0.2, 0.1.0-0.1.1, 0.2.0] or uninstalled
 ├─restricted to versions * by an explicit requirement, leaving only versions [0.0.1-0.0.2, 0.1.0-0.1.1, 0.2.0]
 └─restricted by julia compatibility requirements to versions: uninstalled — no versions left
Stacktrace:
 [1] #propagate_constraints!#61(::Bool, ::Function, ::Pkg.GraphType.Graph, ::Set{Int64}) at C:\cygwin\home\Administrator\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.0\Pkg\src\GraphType.jl:1005
 [2] propagate_constraints! at C:\cygwin\home\Administrator\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.0\Pkg\src\GraphType.jl:946 [inlined]
 [3] #simplify_graph!#121(::Bool, ::Function, ::Pkg.GraphType.Graph, ::Set{Int64}) at C:\cygwin\home\Administrator\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.0\Pkg\src\GraphType.jl:1460
 [4] simplify_graph! at C:\cygwin\home\Administrator\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.0\Pkg\src\GraphType.jl:1460 [inlined] (repeats 2 times)
 [5] resolve_versions!(::Pkg.Types.Context, ::Array{Pkg.Types.PackageSpec,1}, ::Nothing) at C:\cygwin\home\Administrator\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.0\Pkg\src\Operations.jl:373
 [6] resolve_versions! at C:\cygwin\home\Administrator\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.0\Pkg\src\Operations.jl:316 [inlined]
 [7] #add_or_develop#62(::Array{Base.UUID,1}, ::Symbol, ::Function, ::Pkg.Types.Context, ::Array{Pkg.Types.PackageSpec,1}) at C:\cygwin\home\Administrator\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.0\Pkg\src\Operations.jl:1201
 [8] #add_or_develop at .\none:0 [inlined]
 [9] #add_or_develop#13(::Symbol, ::Bool, ::Base.Iterators.Pairs{Union{},Union{},Tuple{},NamedTuple{(),Tuple{}}}, ::Function, ::Pkg.Types.Context, ::Array{Pkg.Types.PackageSpec,1}) at C:\cygwin\home\Administrator\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.0\Pkg\src\API.jl:64
 [10] #add_or_develop at .\none:0 [inlined]
 [11] #add_or_develop#12 at C:\cygwin\home\Administrator\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.0\Pkg\src\API.jl:29 [inlined]
 [12] #add_or_develop at .\none:0 [inlined]
 [13] #add_or_develop#11 at C:\cygwin\home\Administrator\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.0\Pkg\src\API.jl:28 [inlined]
 [14] #add_or_develop at .\none:0 [inlined]
 [15] #add_or_develop#10 at C:\cygwin\home\Administrator\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.0\Pkg\src\API.jl:27 [inlined]
 [16] #add_or_develop at .\none:0 [inlined]
 [17] #add#18 at C:\cygwin\home\Administrator\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.0\Pkg\src\API.jl:69 [inlined]
 [18] add(::String) at C:\cygwin\home\Administrator\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.0\Pkg\src\API.jl:69
 [19] top-level scope at none:0

julia>