Work around JuliaLang/julia#8818

This should fix numerous related errors in other packages

src/colorspaces.jl

@@ -302,6 +302,7 @@ const CVconcrete = (HSV, HSL, XYZ, xyY, Lab, Luv, LCHab, LCHuv, DIN99, DIN99d, D
 const CVparametric = tuple(RGB, CVconcrete...)
 const CVfractional = (RGB, XYZ)
 const CVfloatingpoint = (HSV, HSL, xyY, Lab, Luv, LCHab, LCHuv, DIN99, DIN99d, DIN99o, LMS)
+const CVAlpha = (RGBA, HSVA, HSLA, XYZA, xyYA, LabA, LCHabA, LuvA, LCHuvA, DIN99A, DIN99dA, DIN99oA, LMSA)
 
 for CV in CVparametric
     @eval begin

src/conversions.jl

@@ -688,15 +688,17 @@ convert{C,T}(::Type{AlphaColorValue{C,T}}, c::AlphaColorValue{C,T}) = c
 function convert{C,T,D,U}(::Type{AlphaColorValue{C,T}}, c::AbstractAlphaColorValue{D,U})
     AlphaColorValue{C,T}(convert(C, c.c), c.alpha)
 end
-function convert{D,T}(AC::TypeConstructor, c::AbstractAlphaColorValue{D,T})
-    AlphaColorValue(convert(colortype(AC), c.c), c.alpha)
+for CV in CVAlpha
+    @eval begin
+        function convert{D,T}(AC::Type{$CV}, c::AbstractAlphaColorValue{D,T})
+            AlphaColorValue(convert(colortype($CV), c.c), c.alpha)
+        end
+        convert(AC::Type{$CV}, c::ColorValue) = AlphaColorValue(convert(colortype($CV), c))
+    end
 end
 
 convert{C,T}(::Type{AlphaColorValue{C,T}}, c::ColorValue) =
     AlphaColorValue{C,T}(convert(C, c), one(T))
-function convert(AC::TypeConstructor, c::ColorValue)
-    AlphaColorValue(convert(colortype(AC), c))
-end
 convert{C<:ColorValue,D,T}(::Type{C}, c::AlphaColorValue{D, T}) = convert(C, c.c)
 
 convert(::Type{ARGB32}, c::ARGB32) = c