Package manager: what are "explicit requirements"?

General Usage
4

I’m having similar issues with package requirements. This time it’s StatsBase that causes issues. I also find the message
restricted to versions 0.30.0 by an explicit requirement, leaving only versions 0.30.0 kind of cryptic. Where does this explicit requirement come from and how can I work around the problem. I have not myself set it, so it would be nice if the error message could give me some more pointers. I have tried ] up and the error persists. The full error message is

ERROR: Unsatisfiable requirements detected for package StatsBase [2913bbd2]:
 StatsBase [2913bbd2] log:
 ├─possible versions are: [0.1.0, 0.2.0-0.2.1, 0.2.3-0.2.10, 0.3.0-0.3.13, 0.4.0-0.4.4, 0.5.0-0.5.3, 0.6.0-0.6.16, 0.7.0-0.7.4, 0.8.0-0.8.3, 0.9.0, 0.10.0, 0.11.0-0.11.1, 0.12.0, 0.13.0-0.13.1, 0.14.0-0.14.1, 0.15.0, 0.16.0-0.16.1, 0.17.0, 0.18.0, 0.19.0-0.19.5, 0.20.0-0.20.1, 0.22.0, 0.23.0-0.23.1, 0.24.0, 0.25.0, 0.26.0, 0.27.0, 0.28.0-0.28.1, 0.29.0, 0.30.0] or uninstalled
 ├─restricted to versions 0.30.0 by an explicit requirement, leaving only versions 0.30.0
 └─restricted by compatibility requirements with StatsPlots [f3b207a7] to versions: [0.1.0, 0.2.0-0.2.1, 0.2.3-0.2.10, 0.3.0-0.3.13, 0.4.0-0.4.4, 0.5.0-0.5.3, 0.6.0-0.6.16, 0.7.0-0.7.4, 0.8.0-0.8.3, 0.9.0, 0.10.0, 0.11.0-0.11.1, 0.12.0, 0.13.0-0.13.1, 0.14.0-0.14.1, 0.15.0, 0.16.0-0.16.1, 0.17.0, 0.18.0, 0.19.0-0.19.5, 0.20.0-0.20.1, 0.22.0, 0.23.0-0.23.1, 0.24.0, 0.25.0, 0.26.0, 0.27.0, 0.28.0-0.28.1, 0.29.0] — no versions left
   └─StatsPlots [f3b207a7] log:
     ├─possible versions are: 0.10.0-0.10.2 or uninstalled
     └─restricted to versions * by an explicit requirement, leaving only versions 0.10.0-0.10.2

My workaround that works locally, but not on travis, was

  1. rm StatsBase
  2. rm Distributions
  3. add StatsPlots
  4. add Distributions
  5. add StatsBase
    which resulted in
(LowLevelParticleFilters) pkg> st
Project LowLevelParticleFilters v0.1.0
    Status `~/.julia/dev/LowLevelParticleFilters/Project.toml`
  [31c24e10] + Distributions v0.18.0
  [50d2b5c4] + Lazy v0.13.2
  [90014a1f] + PDMats v0.9.6
  [d96e819e] + Parameters v0.10.3
  [90137ffa] + StaticArrays v0.10.3
  [2913bbd2] + StatsBase v0.29.0
  [f3b207a7] + StatsPlots v0.10.2
  [6310b701] + Yeppp v0.4.0
  [37e2e46d] + LinearAlgebra 
  [9a3f8284] + Random 
  [10745b16] + Statistics

It would be very nice to know that the explicit requirement comes from me doing add StatsBase (which added v0.30.0) before I added the other packages (one of which required <= v0.29.0), and that this workaround existed.