[maleadt]

oneArray rejected isbits-union element types (Union{Int,Nothing}, Union{Missing,Bool}, ...) with "oneArray does not yet support isbits-union arrays". This adds support, following the layout and design of JuliaGPU/AMDGPU.jl#1182 (see also JuliaGPU/AMDGPU.jl#1171).

Layout is the same as Array's: the data, aligned_sizeof(T) bytes per element, followed by one selector byte per element. The device side already had the union load/store code; what was missing is on the host:

• maxsize locates an array's selector bytes. GPUArrays.derive advances it by the element offset, since the byte offset can't recover that for unions of singletons (Union{Nothing,Missing}), and kernels get maxsize - offset so that views use their own selector bytes.
• Every copy path copies the selector bytes too: uploads, downloads, device-to-device, the host-pointer paths for shared and host memory, and resize!. Union uploads aren't staged (they synchronize instead), as the selector bytes would need a staged copy of their own.
• Scalar indexing of host-accessible union arrays goes through a copy instead of unsafe_load; unsafe_wrap(Array, ...) rejects unions.
• On the device, Const arrays (which AcceleratedKernels' reductions use) load unions through their selector byte, and structs with a union field no longer take the union path, which read a selector byte past the end of the array.
• Element sizes use aligned_sizeof, which differs from sizeof for e.g. Union{Int64,NTuple{3,Int32}} (16 vs. 12 bytes). On Julia 1.10 it's wrapped in a @generated function, as Base.aligned_sizeof doesn't fold to a constant in device code there.

This also enables the union tests in GPUArrays' testsuite (reinterpret of a union array, any/all on Union{Missing,Bool}), which check for support at run time.

Not addressed: SPIRVIntrinsics' emit_localmemory sizes local memory as len * sizeof(T) plus the selector bytes, which is too small for oneLocalArrays of unions like Union{Int64,NTuple{3,Int32}} whose size isn't a multiple of their alignment.

[codecov]

Codecov Report

❌ Patch coverage is 96.42857% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.60%. Comparing base (7020223) to head (262e6c9).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/array.jl	95.55%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #735      +/-   ##
==========================================
+ Coverage   85.08%   85.60%   +0.51%     
==========================================
  Files          53       52       -1     
  Lines        4660     4550     -110     
==========================================
- Hits         3965     3895      -70     
+ Misses        695      655      -40     

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