improve errors, add tests

src/functionlenses.jl

@@ -49,14 +49,16 @@ set(obj, ::typeof(Base.splat(=>)), val::Pair) = @set Tuple(obj) = Tuple(val)
 
 set(obj, ::typeof(getproperties), val::NamedTuple) = setproperties(obj, val)
 
-set(x::Union{Tuple,NamedTuple}, ::typeof(propertynames), names) =
+function set(x::Union{Tuple,NamedTuple}, ::typeof(propertynames), names)
+    length(names) == length(x) || throw(ArgumentError("Got $(length(names)) for $(length(x)) properties"))
     if eltype(names) === Symbol
         NamedTuple{names}(Tuple(x))
     elseif eltype(names) <: Integer && names == ntuple(identity, length(names))
         Tuple(x)
     else
-        error("invalid property names: $names")
+        throw(ArgumentError("invalid property names: $names"))
     end
+end
 
 ################################################################################
 ##### eltype

test/test_functionlenses.jl

@@ -110,6 +110,15 @@ end
 
     test_getset_laws(Accessors.getproperties, 1+2im, (im=4., re=3.), (re=5, im=6); cmp=cmp)
 
+    @testset for obj in [(10, 20.), (x=10, y=20.)]
+        @test (@set propertynames(obj) == 1:2) === obj
+        @test (@set propertynames(obj) == (:a, :b)) === (a=10, b=20.)
+        @test_throws Exception (@set propertynames(obj) == 1:3)
+        @test_throws Exception (@set propertynames(obj) == (2, 3))
+        @test_throws Exception (@set propertynames(obj) == (:a, :b, :c))
+        @test_throws Exception (@set propertynames(obj) == (1, :b))
+        @test_throws Exception (@set propertynames(obj) == ("a", "b"))
+    end
     test_getset_laws(propertynames, (10, 20.), (:a, :b), (1, 2); cmp=(===))
     test_getset_laws(propertynames, (10, 20.), (1, 2), (:a, :b); cmp=(===))
     test_getset_laws(propertynames, (a=10, b=20.), (:x, :y), (1, 2); cmp=(===))