defining unit relative to prefixed unit causes problems

[ggggggggg]

I defined a Liter unit based on 1000cm^3, and it seemed to work most of the time, but then failed on some operations. If I instead define it based on m^3, then it seems to work as expected. I added the following lines to Defaults.jl to test this:

@unit L     "L"         Liter       (1//1000)*m^3           true
@unit altL  "altL"      altLiter    1000*cm^3               true
@unit arb   "arb"       Arb         1nA                     true

And then did the following tests at the REPL that all work.

julia> a = 1u"altL"
1 altL
julia> a = 1u"maltL"
1 maltL
julia> dimension(a)
𝐋^3

But then trying to get back to a unitless number fails

julia> Float64(1u"altL/cm^3")
ERROR: DomainError:
Cannot raise an integer x to a negative power -n. 
Make x a float by adding a zero decimal (e.g. 2.0^-n instead of 2^-n), or write 1/x^n, float(x)^-n, or (x//1)^-n.
 in power_by_squaring(::Int64, ::Int64) at ./intfuncs.jl:118
 in basefactorhelper(::Float64, ::Int64, ::Int64, ::Rational{Int64}) at /Users/oneilg/.julia/v0.5/Unitful/src/Unitful.jl:268
 in map(::Unitful.#basefactor, ::Tuple{Unitful.Unit{:Meter},Unitful.Unit{:altLiter}}) at ./tuple.jl:93
 in basefactor(...) at /Users/oneilg/.julia/v0.5/Unitful/src/Unitful.jl:276
 in convfact(...) at /Users/oneilg/.julia/v0.5/Unitful/src/Conversion.jl:85
 in convert at /Users/oneilg/.julia/v0.5/Unitful/src/Conversion.jl:183 [inlined]
 in Float64(::Unitful.Quantity{Int64,Unitful.Dimensions{()},Unitful.Units{(Unitful.Unit{:Meter}(-2,-3//1),Unitful.Unit{:altLiter}(0,1//1)),Unitful.Dimensions{()}}}) at ./sysimg.jl:53

The version based on m^3 works for going to a unitless number

julia> Float64(1u"L/cm^3")
1000.0

And arb (based on nA) shows the same error as altL

julia> Float64(1u"arb/A")
ERROR: DomainError:
Cannot raise an integer x to a negative power -n. 
Make x a float by adding a zero decimal (e.g. 2.0^-n instead of 2^-n), or write 1/x^n, float(x)^-n, or (x//1)^-n.
 in power_by_squaring(::Int64, ::Int64) at ./intfuncs.jl:118
 in basefactorhelper(::Float64, ::Int64, ::Int64, ::Rational{Int64}) at /Users/oneilg/.julia/v0.5/Unitful/src/Unitful.jl:268
 in map(::Unitful.#basefactor, ::Tuple{Unitful.Unit{:Ampere},Unitful.Unit{:Arb}}) at ./tuple.jl:93
 in basefactor(...) at /Users/oneilg/.julia/v0.5/Unitful/src/Unitful.jl:276
 in convfact(...) at /Users/oneilg/.julia/v0.5/Unitful/src/Conversion.jl:85
 in convert at /Users/oneilg/.julia/v0.5/Unitful/src/Conversion.jl:183 [inlined]
 in Float64(::Unitful.Quantity{Int64,Unitful.Dimensions{()},Unitful.Units{(Unitful.Unit{:Ampere}(0,-1//1),Unitful.Unit{:Arb}(0,1//1)),Unitful.Dimensions{()}}}) at ./sysimg.jl:53

[ajkeller34]

Fixed on master and added a test for this condition. The fix will be included in next release or you can Pkg.checkout("Unitful") if you want to be on master.