prevent get_backend from overflowing the stack (#602)

Co-authored-by: Valentin Churavy <v.churavy@gmail.com>

Author: nsajko

src/KernelAbstractions.jl

@@ -511,7 +511,13 @@ Get a [`Backend`](@ref) instance suitable for array `A`.
 function get_backend end
 
 # Should cover SubArray, ReshapedArray, ReinterpretArray, Hermitian, AbstractTriangular, etc.:
-get_backend(A::AbstractArray) = get_backend(parent(A))
+function get_backend(A::AbstractArray)
+    P = parent(A)
+    if P isa typeof(A)
+        throw(ArgumentError("Implement `KernelAbstractions.get_backend(::$(typeof(A)))`"))
+    end
+    return get_backend(P)
+end
 
 # Define:
 #   adapt_storage(::Backend, a::Array) = adapt(BackendArray, a)

test/test.jl

@@ -7,6 +7,9 @@ using Adapt
 
 identity(x) = x
 
+struct UnknownAbstractVector <: AbstractVector{Float32}  # issue #588
+end
+
 function unittest_testsuite(Backend, backend_str, backend_mod, BackendArrayT; skip_tests = Set{String}())
     @conditional_testset "partition" skip_tests begin
         backend = Backend()
@@ -80,6 +83,7 @@ function unittest_testsuite(Backend, backend_str, backend_mod, BackendArrayT; sk
         @test @inferred(KernelAbstractions.get_backend(view(A, 2:4, 1:3))) isa backendT
         @test @inferred(KernelAbstractions.get_backend(Diagonal(x))) isa backendT
         @test @inferred(KernelAbstractions.get_backend(Tridiagonal(A))) isa backendT
+        @test_throws ArgumentError KernelAbstractions.get_backend(UnknownAbstractVector())  # issue #588
     end
 
     @conditional_testset "sparse" skip_tests begin