allow vcat on single ChainedVector

src/SentinelArrays.jl

@@ -58,6 +58,9 @@ mutable struct SentinelArray{T, N, S, V, A <: AbstractArray{T, N}} <: AbstractAr
 end
 
 Base.parent(A::SentinelArray) = A.data
+function Base.unaliascopy(A::SentinelArray)
+    return SentinelArray(Base.unaliascopy(parent(A)), A.sentinel, A.value)
+end
 # users of pointer should be careful because it's the raw storage data, which doesn't respect the sentinel semantics
 Base.pointer(A::SentinelArray) = pointer(parent(A))
 

src/chainedvector.jl

@@ -477,7 +477,7 @@ function Base.copy(A::ChainedVector{T}) where {T}
 end
 
 function Base.unaliascopy(x::ChainedVector{T, A}) where {T, A}
-    arrays = map(copy, x.arrays)
+    arrays = map(Base.unaliascopy, x.arrays)
     return ChainedVector{T, A}(arrays, copy(x.inds))
 end
 
@@ -630,6 +630,7 @@ Base.@propagate_inbounds function Base.insert!(A::ChainedVector{T, AT}, i::Integ
 end
 
 function Base.vcat(A::ChainedVector{T, AT}, arrays::ChainedVector{T, AT}...) where {T, AT <: AbstractVector{T}}
+    isempty(arrays) && return Base.unaliascopy(A)
     newarrays = vcat(A.arrays, map(x->x.arrays, arrays)...)
     n = length(A.inds)
     inds = Vector{Int}(undef, n + sum(x->length(x.inds), arrays))

test/chainedvector.jl

@@ -1,3 +1,19 @@
+function test_unaliased_vcat(x, replacement)
+    before = collect(Iterators.map(identity, x))
+    y = @inferred vcat(x)
+    @test isequal(y, x)
+    @test typeof(y) === typeof(x)
+    @test y !== x
+    @test y.arrays !== x.arrays
+    @test y.inds !== x.inds
+    if !isempty(y)
+        y[1] = replacement
+        @test isequal(collect(Iterators.map(identity, x)), before)
+    end
+    empty!(y)
+    @test isequal(collect(Iterators.map(identity, x)), before)
+end
+
 @testset "ChainedVector" begin
 
     x = ChainedVector([[1,2,3], [4,5,6], [7,8,9,10]])
@@ -58,6 +74,15 @@
     x = ChainedVector([[1,2,3], [4,5,6], [7,8,9,10]])
     y = ChainedVector([[11,12,13], [14,15,16], [17,18,19,20]])
 
+    @testset "single-input vcat" begin
+        test_unaliased_vcat(ChainedVector(Vector{Int}[]), 1)
+        test_unaliased_vcat(ChainedVector([[1]]), 2)
+        test_unaliased_vcat(ChainedVector(AbstractVector[[1], Float32[2]]), 3.0f0)
+        test_unaliased_vcat(ChainedVector([Union{Missing, Int}[1, missing]]), 2)
+        test_unaliased_vcat(ChainedVector([SentinelArray(Union{Missing, Int}[1, missing])]), 2)
+        test_unaliased_vcat(ChainedVector([ChainedVector([[1]]), ChainedVector([[2]])]), 3)
+    end
+
     z = vcat(x, y)
     @test length(z) == 20
     @test z == 1:20