Attempt to allow preferunits to work with non-pure units

src/Unitful.jl

@@ -53,7 +53,7 @@ end
 const basefactors = _basefactors(Unitful)
 
 include("types.jl")
-const promotion = Dict{Symbol,Unit}()
+const promotion = Dict{Symbol,FreeUnits}()
 
 include("user.jl")
 include("utils.jl")

src/units.jl

@@ -302,6 +302,6 @@ factory defaults, this function will return a product of powers of base SI units
 (as [`Unitful.FreeUnits`](@ref)).
 """
 @generated function upreferred(x::Dimensions{D}) where {D}
-    u = *(FreeUnits{((Unitful.promotion[name(z)]^z.power for z in D)...,),()}())
+    u = prod((Unitful.NoUnits, (Unitful.promotion[name(z)]^z.power for z in D)...))
     :($u)
 end

src/user.jl

@@ -310,7 +310,28 @@ function preferunits(u0::Units, u::Units...)
             "For instance, it should not be used with units of dimension 𝐋^2.")
         end
         y = typeof(dim).parameters[1][1]
-        promotion[name(y)] = typeof(unit).parameters[1][1]
+        promotion[name(y)] = unit
+    end
+
+    # Let's run a quick check for anything where, for instance, current was defined is C/ms and time
+    # was defined in ns. We'll give a warning if this is the case so the user knows they may get
+    # unexpected (though still correct) results. There may be cases where people would want to do this,
+    # so we won't stop them, just let them know they're doing it.
+    ulist = (typeof(i[2]).parameters[1] for i in promotion)
+    check = Dict{Unitful.Dimensions, Unit}()
+    for a in ulist
+        ulistA = (i^(1/i.power) for i in a)
+        for i in ulistA
+            k = dimension(i)
+            if haskey(check, k)
+                if i!=check[k]
+                    @warn "Preferred units contain complex units based on units of the same dimension but different scales: "*string(i)*" and "*string(check[k])*
+                    "\nThis may be intentional, but otherwise could lead to redundant units, such as ms/s or kg/g"
+                end
+            else
+                check[k] = i
+            end
+        end
     end
 
     nothing

test/runtests.jl

@@ -14,7 +14,8 @@ import Unitful:
     J, A, N, mol, V,
     mW, W,
     dB, dB_rp, dB_p, dBm, dBV, dBSPL, Decibel,
-    Np, Np_rp, Np_p, Neper
+    Np, Np_rp, Np_p, Neper,
+    C
 
 import Unitful: 𝐋, 𝐓, 𝐍, 𝚯
 
@@ -320,6 +321,15 @@ Unitful.uconvert(U::Unitful.Units, q::QQQ) = uconvert(U, Quantity(q.val, cm))
 
 @testset "Promotion" begin
     @testset "> Unit preferences" begin
+        # Should warn on possible redundant units issue (ms and s)
+        @test_logs (:warn, r"^Preferred units contain complex units") Unitful.preferunits(C/ms)
+        # Test for wacky prefered units functionality
+        Unitful.preferunits(C/s)
+        @test @inferred(upreferred(u"V/m")) == u"kg*m*C^-1*s^-2"
+        @test dimension(upreferred(u"V/m")) == dimension(u"V/m")
+        # Reset prefered units to default
+        Unitful.preferunits(A)
+
         # Only because we favor SI, we have the following:
         @test @inferred(upreferred(N)) === kg*m/s^2
         @test @inferred(upreferred(dimension(N))) === kg*m/s^2