julia> versioninfo()
Julia Version 1.6.7
Commit 3b76b25b64 (2022-07-19 15:11 UTC)
Platform Info:
  OS: macOS (x86_64-apple-darwin21.4.0)
  CPU: Apple M1 Pro
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-11.0.1 (ORCJIT, westmere)

julia> S =  wrapdims(Symmetric(rand(Int8,3,3)), 'a':'c', 10:10:30)
2-dimensional KeyedArray(...) with keys:
↓   3-element StepRange{Char,...}
→   3-element StepRange{Int64,...}
And data, 3×3 Symmetric{Int8, Matrix{Int8}}:
         (10)  (20)  (30)
  ('a')   -37  -122    -2
  ('b')  -122    90  -117
  ('c')    -2  -117   -12

julia> A = copy(S)
2-dimensional KeyedArray(...) with keys:
↓   3-element StepRange{Char,...}
→   3-element StepRange{Int64,...}
And data, 3×3 Symmetric{Int8, Matrix{Int8}}:
         (10)  (20)  (30)
  ('a')   -37  -122    -2
  ('b')  -122    90  -117
  ('c')    -2  -117   -12

julia> copyto!(A, S)
ERROR: ArgumentError: Cannot set a non-diagonal index in a symmetric matrix
Stacktrace:
 [1] setindex!
   @ /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.6/LinearAlgebra/src/symmetric.jl:225 [inlined]
 [2] setindex!
   @ ~/.julia/packages/AxisKeys/DiXQB/src/struct.jl:132 [inlined]
 [3] copyto_unaliased!(deststyle::IndexCartesian, dest::KeyedArray{Int8, 2, Symmetric{Int8, Matrix{Int8}}, Tuple{StepRange{Char, Int64}, StepRange{Int64, Int64}}}, srcstyle::IndexCartesian, src::KeyedArray{Int8, 2, Symmetric{Int8, Matrix{Int8}}, Tuple{StepRange{Char, Int64}, StepRange{Int64, Int64}}})
   @ Base ./abstractarray.jl:984
 [4] copyto!(dest::KeyedArray{Int8, 2, Symmetric{Int8, Matrix{Int8}}, Tuple{StepRange{Char, Int64}, StepRange{Int64, Int64}}}, src::KeyedArray{Int8, 2, Symmetric{Int8, Matrix{Int8}}, Tuple{StepRange{Char, Int64}, StepRange{Int64, Int64}}})
   @ Base ./abstractarray.jl:950
 [5] top-level scope
   @ REPL[14]:1

julia> @which copyto!(A, S)
copyto!(dest::AbstractArray, src::AbstractArray) in Base at abstractarray.jl:947