define propertynames() to be consistent with getproperty()

src/MVector.jl

@@ -18,6 +18,9 @@
 let dimension_names = QuoteNode.([:x, :y, :z, :w])
     body = :(getfield(v, name))
     for (i,dim_name) in enumerate(dimension_names)
+        @eval @inline Base.propertynames(v::Union{SVector{$i},MVector{$i}}, private::Bool = false) =
+            private ? ($(first(dimension_names, i)...), :data) : ($(first(dimension_names, i)...),)
+
         body = :(name === $(dimension_names[i]) ? getfield(v, :data)[$i] : $body)
         @eval @inline function Base.getproperty(v::Union{SVector{$i},MVector{$i}},
                                                 name::Symbol)
@@ -33,3 +36,6 @@
         end
     end
 end
+
+# for longer S/MVectors and other S/MArrays, the only property is data, and it's private
+Base.propertynames(::Union{SArray,MArray}, private::Bool = false) = private ? (:data,) : ()

test/SVector.jl

@@ -151,12 +151,19 @@
     end
 
     @testset "Named field access - getproperty" begin
+        @test propertynames(SA[1,2,3,4,5]) == ()
+        @test propertynames(SA[1,2,3,4,5], true) == (:data,)
+        @test propertynames(SA[1 2; 3 4], true) == (:data,)
         v4 = SA[10,20,30,40]
+        @test propertynames(v4) == (:x, :y, :z, :w)
+        @test propertynames(v4, true) == (:x, :y, :z, :w, :data)
         @test v4.x == 10
         @test v4.y == 20
         @test v4.z == 30
         @test v4.w == 40
         v2 = SA[10,20]
+        @test propertynames(v2) == (:x, :y)
+        @test propertynames(v2, true) == (:x, :y, :data)
         @test v2.x == 10
         @test v2.y == 20
         @test_throws ErrorException v2.z