julia> using CUDA, LinearAlgebra

julia> CUDA.allowscalar(false)

julia> U = cu(UpperTriangular(
           [1.0 0.0;
            0.0 1.0]
       ))
2×2 UpperTriangular{Float32, CuArray{Float32, 2}}:
 1.0  0.0
  ⋅   1.0

julia> copyto!(Σ, U')
ERROR: scalar getindex is disallowed
Stacktrace:
  [1] error(s::String)
    @ Base ./error.jl:33
  [2] assertscalar(op::String)
    @ GPUArrays ~/.julia/packages/GPUArrays/4n0iS/src/host/indexing.jl:62
  [3] getindex(::CuArray{Float32, 2}, ::Int64, ::Int64)
    @ GPUArrays ~/.julia/packages/GPUArrays/4n0iS/src/host/indexing.jl:104
  [4] getindex
    @ /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.6/LinearAlgebra/src/adjtrans.jl:203 [inlined]
  [5] getindex
    @ /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.6/LinearAlgebra/src/triangular.jl:222 [inlined]
  [6] _getindex
    @ ./abstractarray.jl:1214 [inlined]
  [7] getindex
    @ ./abstractarray.jl:1170 [inlined]
  [8] iterate
    @ ./abstractarray.jl:1096 [inlined]
  [9] iterate
    @ ./abstractarray.jl:1094 [inlined]
 [10] copyto_unaliased!(deststyle::IndexLinear, dest::CuArray{Float32, 2}, srcstyle::IndexCartesian, src::LowerTriangular{Float32, Adjoint{Float32, CuArray{Float32, 2}}})
    @ Base ./abstractarray.jl:975
 [11] copyto!(dest::CuArray{Float32, 2}, src::LowerTriangular{Float32, Adjoint{Float32, CuArray{Float32, 2}}})
    @ Base ./abstractarray.jl:950
 [12] top-level scope
    @ REPL[11]:1
 [13] top-level scope
    @ ~/.julia/packages/CUDA/LTbUr/src/initialization.jl:81
    
julia> @which copyto!(Σ, U')
copyto!(dest::AbstractArray, src::AbstractArray) in Base at abstractarray.jl:947

julia> typeof(U')
LowerTriangular{Float32, Adjoint{Float32, CuArray{Float32, 2}}}