Deprecate GPU as an alias of Backend instead of removing it

docs/src/index.md

@@ -141,10 +141,8 @@ end
   the CPU backend became an OpenCL backend meant every backend. Use
   `KernelAbstractions.@device_code_llvm` and `KernelAbstractions.@device_code_typed` instead,
   which report on the code a backend actually generates; see [Reflection](@ref).
-- `KernelAbstractions.GPU` has been removed: it didn't mean GPU hardware (the `CPU` backend
-  was a subtype), and backends now subtype `KernelAbstractions.Backend` directly. Code
-  dispatching on `::GPU` should dispatch on `::Backend`, on concrete backend types, or on a
-  capability such as `KernelAbstractions.supports_float64`.
+- `KernelAbstractions.GPU` is deprecated: and backends should now subtype `KernelAbstractions.Backend` directly. `GPU` is now an alias of `Backend` and will be removed in a future release. Code dispatching on `::GPU` should dispatch on `::Backend`, on concrete
+  backend types, or on a capability such as `KernelAbstractions.supports_float64`.
 - An exception thrown in a kernel on the `CPU` backend is reported as a
   `KernelAbstractions.POCL.KernelException` when the kernel has completed, like on GPU
   backends, and no longer as the original exception (e.g., a `BoundsError`). Depending on

src/KernelAbstractions.jl

@@ -18,6 +18,10 @@ using KernelInterface: KernelInterface, Backend, get_backend, functional, synchr
 import KernelInterface as KI
 export KernelInterface
 
+# `GPU` is deprecated in 0.10:  and backends ends now subtype `Backend` directly. Keep the name working for code written
+# against 0.9, both `x::GPU` and `struct MyBackend <: GPU`.
+Base.@deprecate_binding GPU Backend
+
 """
     @kernel function f(args) end
 

test/runtests.jl

@@ -423,3 +423,9 @@ struct NewBackend <: KernelAbstractions.Backend end
 
     @test KernelAbstractions.functional(backend) === missing
 end
+
+@testset "Deprecated GPU alias" begin
+    @test KernelAbstractions.GPU === KernelAbstractions.Backend
+    @test CPU() isa KernelAbstractions.GPU
+    @test NewBackend <: KernelAbstractions.GPU
+end