`Base.show(io::IO, x::Level)` error

[giopaglia]

Hi, thank you for this wonderful package!

Here's the thing, I can construct a Level:

y = Unitful.Level{Unitful.Decibel,1}(2);

But I can't show it:

julia> y
Error showing value of type Level{Unitful.LogInfo{:Decibel, 10, 10}, 1, Int64}:
ERROR: undefined behavior. Please file an issue with the code needed to reproduce.
Stacktrace:
  [1] error(s::String)
    @ Base ./error.jl:35
  [2] isrootpower_dim(y::Unitful.Dimensions{()})
    @ Unitful ~/.julia/packages/Unitful/dHMk1/src/logarithm.jl:167
  [3] isrootpower(y::Int64)
    @ Unitful ~/.julia/packages/Unitful/dHMk1/src/logarithm.jl:166
  [4] tolog(L::Type{Unitful.LogInfo{:Decibel, 10, 10}}, S::Int64, x::Float64)
    @ Unitful ~/.julia/packages/Unitful/dHMk1/src/logarithm.jl:82
  [5] ustrip(x::Level{Unitful.LogInfo{:Decibel, 10, 10}, 1, Int64})
    @ Unitful ~/.julia/packages/Unitful/dHMk1/src/logarithm.jl:162
  [6] show(io::IOContext{Base.TTY}, x::Level{Unitful.LogInfo{:Decibel, 10, 10}, 1, Int64})
    @ Unitful ~/.julia/packages/Unitful/dHMk1/src/logarithm.jl:314
  [7] show(io::IOContext{Base.TTY}, ::MIME{Symbol("text/plain")}, x::Level{Unitful.LogInfo{:Decibel, 10, 10}, 1, Int64})
    @ Base.Multimedia ./multimedia.jl:47
...

A similar issue exists with:

x1 = @dB (-2u"V")/1u"V";

where I can construct it, but cannot show it.
Here, the issue is probably due to logarithmic scales not allowing negative values. In fact, this works perfectly:

julia> x2 = @dB (2u"V")/1u"V"
6.020599913279624 dBV

But then, the issue is that that x1 construction shouldn´t probably be allowed at all.

[sostock]

See #788 for an explanation why showing y does not work (and why it could be considered breaking to make this an error).