Reject transformations on SubDataFrame with copycols=false

src/abstractdataframe/selection.jl

@@ -1820,22 +1820,27 @@ function manipulate(dfv::SubDataFrame, @nospecialize(args...); copycols::Bool, k
     else
         # we do not support transformations here
         # newinds contains only indexing; making it Vector{Any} avoids some compilation
-        newinds = []
+        newinds = Any[]
         seen_single_column = Set{Int}()
         for ind in args
+            if ind isa Pair || ind isa AbstractVecOrMat{<:Pair}
+                throw(ArgumentError(
+                "Transformations are not supported for SubDataFrame with copycols=false. " *
+                "Only column selection is allowed."
+                ))
+           end
+            ind_idx = index(dfv)[ind]
             if ind isa ColumnIndex
-                ind_idx = index(dfv)[ind]
                 if ind_idx in seen_single_column
-                    throw(ArgumentError("selecting the same column multiple times " *
-                                        "using Symbol, string or integer is not allowed " *
-                                        "($ind was passed more than once"))
-                else
-                    push!(seen_single_column, ind_idx)
+                    throw(ArgumentError(
+                        "selecting the same column multiple times " *
+                        "using Symbol, string or integer is not allowed " *
+                        "($ind was passed more than once)"
+                    ))
                 end
-            else
-                newind = index(dfv)[ind]
-                push!(newinds, newind)
-            end
+                push!(seen_single_column, ind_idx)
+             end
+           push!(newinds, ind_idx)
         end
         return view(dfv, :, Cols(newinds...))
     end

test/utils.jl

@@ -166,3 +166,33 @@ end
 end
 
 end # module
+
+@testset "SubDataFrame copycols=false rejects transformations" begin
+    df = DataFrame(a = [1, 2, 3])
+    v = @view df[1:2, :]
+
+    err = try
+        select(v, :a => ByRow(x -> x + 1) => :a; copycols=false)
+    catch e
+        e
+    end
+
+    @test err isa ArgumentError
+    @test occursin("copycols=false", sprint(showerror, err))
+end
+
+@testset "SubDataFrame copycols=false column selection" begin
+    df = DataFrame(a = [1, 2, 3], b = [1, 2, 3])
+    v = @view df[1:2, :]
+
+    res = select(v, :a, :b, copycols=false)
+
+    @test size(res) == (2, 2)
+    @test names(res) == ["a", "b"]
+
+    @test res.a == v.a
+    @test res.b == v.b
+
+    @test parent(res.a) === parent(v.a)
+    @test parent(res.b) === parent(v.b)
+end