Fix findall with an empty CuArray of Bool (#2554)

Author: amontoison

src/indexing.jl

@@ -27,7 +27,7 @@ function Base.findall(bools::AnyCuArray{Bool})
     I = keytype(bools)
     indices = cumsum(reshape(bools, prod(size(bools))))
 
-    n = @allowscalar indices[end]
+    n = isempty(indices) ? 0 : @allowscalar indices[end]
     ys = CuArray{I}(undef, n)
 
     if n > 0

test/base/array.jl

@@ -478,10 +478,14 @@ end
 
 @testset "findall" begin
     # 1D
+    @test testf(x->findall(x), rand(Bool, 0))
     @test testf(x->findall(x), rand(Bool, 100))
     @test testf(x->findall(y->y>0.5, x), rand(100))
 
     # ND
+    let x = rand(Bool, 0, 0)
+      @test findall(x) == Array(findall(CuArray(x)))
+    end
     let x = rand(Bool, 10, 10)
       @test findall(x) == Array(findall(CuArray(x)))
     end