Avoid piracy in cat (#254)

* Avoid piracy in cat

* Add test

Author: jishnub

src/structarray.jl

@@ -445,10 +445,11 @@ end
 
 for op in [:cat, :hcat, :vcat]
     @eval begin
-        function Base.$op(args::StructArray...; kwargs...)
+        function Base.$op(arg1::StructArray, argsrest::StructArray...; kwargs...)
+            args = (arg1, argsrest...)
             f = key -> $op((getproperty(t, key) for t in args)...; kwargs...)
             T = mapreduce(eltype, promote_type, args)
-            StructArray{T}(map(f, propertynames(args[1])))
+            StructArray{T}(map(f, propertynames(arg1)))
         end
     end
 end

test/runtests.jl

@@ -586,6 +586,10 @@ end
     horizontal_concat = StructArray{Pair{Int, String}}(([3 1; 5 6], ["a" "a"; "b" "b"]))
     @test cat(t, t2; dims=2)::StructArray == horizontal_concat == hcat(t, t2)
     @test hcat(t, t2) isa StructArray
+
+    # check that cat(dims=1) doesn't commit type piracy (#254)
+    # We only test that this works, the return value is immaterial
+    @test cat(dims=1) == vcat()
 end
 
 f_infer() = StructArray{ComplexF64}((rand(2,2), rand(2,2)))