Add offset in hvncat dimension calculation to fix issue with 0-length elements in first dimension (#58881)

Author: BioTurboNick

base/abstractarray.jl

@@ -2567,16 +2567,23 @@ function _typed_hvncat_dims(::Type{T}, dims::NTuple{N, Int}, row_first::Bool, as
     end
 
     # discover number of rows or columns
+    # d1 dimension is increased by 1 to appropriately handle 0-length arrays
     for i ∈ 1:dims[d1]
         outdims[d1] += cat_size(as[i], d1)
     end
 
+    # adjustment to handle 0-length arrays
+    first_dim_zero = outdims[d1] == 0
+    if first_dim_zero
+        outdims[d1] = dims[d1]
+    end
+
     currentdims = zeros(Int, N)
     blockcount = 0
     elementcount = 0
     for i ∈ eachindex(as)
         elementcount += cat_length(as[i])
-        currentdims[d1] += cat_size(as[i], d1)
+        currentdims[d1] += first_dim_zero ? 1 : cat_size(as[i], d1)
         if currentdims[d1] == outdims[d1]
             currentdims[d1] = 0
             for d ∈ (d2, 3:N...)
@@ -2604,6 +2611,10 @@ function _typed_hvncat_dims(::Type{T}, dims::NTuple{N, Int}, row_first::Bool, as
             throw(DimensionMismatch("argument $i has too many elements along axis $d1"))
         end
     end
+    # restore 0-length adjustment
+    if first_dim_zero
+        outdims[d1] = 0
+    end
 
     outlen = prod(outdims)
     elementcount == outlen ||

test/abstractarray.jl

@@ -831,6 +831,9 @@ function test_cat(::Type{TestAbstractArray})
     r = rand(Float32, 56, 56, 64, 1);
     f(r) = cat(r, r, dims=(3,))
     @inferred f(r);
+
+    #58866 - ensure proper dimension calculation for 0-dimension elements
+    @test [zeros(1, 0) zeros(1,0); zeros(0,0) zeros(0, 0)] == Matrix{Float64}(undef, 1, 0)
 end
 
 function test_ind2sub(::Type{TestAbstractArray})
@@ -1743,6 +1746,9 @@ using Base: typed_hvncat
     @test ["A";;"B";;"C";;"D"] == ["A" "B" "C" "D"]
     @test ["A";"B";;"C";"D"] == ["A" "C"; "B" "D"]
     @test [["A";"B"];;"C";"D"] == ["A" "C"; "B" "D"]
+
+    #58866 - ensure proper dimension calculation for 0-dimension elements
+    @test [zeros(1, 0) zeros(1,0);;; zeros(0,0) zeros(0, 0)] == Array{Float64, 3}(undef, 1, 0, 0)
 end
 
 @testset "stack" begin