deleteat!: Handle unassigned array elements (#43941)

This function was erroring on unassigned array elements. This is
the minimal fix to resolve that issue. I tried something more
generic, but it turns out all of our Array code basically assumes
that all elements are assigned. Even basic things like `==`, `hash`,
etc. error for arrays with unassigned elements. I think we may want
to strongly consider removing the ability to unassign elements in
arrays in favor of Union{Nothing, T} arrays, which our existing code
handles fine and should be able to be made to have equivalent performance.

base/array.jl

@@ -1531,14 +1531,31 @@ deleteat!(a::Vector, inds::AbstractVector) = _deleteat!(a, to_indices(a, (inds,)
 
 struct Nowhere; end
 push!(::Nowhere, _) = nothing
+_growend!(::Nowhere, _) = nothing
+
+@inline function _push_deleted!(dltd, a::Vector, ind)
+ if @inbounds isassigned(a, ind)
+ push!(dltd, @inbounds a[ind])
+ else
+ _growend!(dltd, 1)
+ end
+end
+
+@inline function _copy_item!(a::Vector, p, q)
+ if @inbounds isassigned(a, q)
+ @inbounds a[p] = a[q]
+ else
+ _unsetindex!(a, p)
+ end
+end
 
 function _deleteat!(a::Vector, inds, dltd=Nowhere())
  n = length(a)
  y = iterate(inds)
  y === nothing && return a
  (p, s) = y
  checkbounds(a, p)
- push!(dltd, @inbounds a[p])
+ _push_deleted!(dltd, a, p)
  q = p+1
  while true
  y = iterate(inds, s)
@@ -1552,14 +1569,14 @@ function _deleteat!(a::Vector, inds, dltd=Nowhere())
  end
  end
  while q < i
- @inbounds a[p] = a[q]
+ _copy_item!(a, p, q)
  p += 1; q += 1
  end
- push!(dltd, @inbounds a[i])
+ _push_deleted!(dltd, a, i)
  q = i+1
  end
  while q <= n
- @inbounds a[p] = a[q]
+ _copy_item!(a, p, q)
  p += 1; q += 1
  end
  _deleteend!(a, n-p+1)
@@ -1572,7 +1589,7 @@ function deleteat!(a::Vector, inds::AbstractVector{Bool})
  length(inds) == n || throw(BoundsError(a, inds))
  p = 1
  for (q, i) in enumerate(inds)
- @inbounds a[p] = a[q]
+ _copy_item!(a, p, q)
  p += !i
  end
  _deleteend!(a, n-p+1)

test/arrayops.jl

@@ -1516,6 +1516,11 @@ end
  @test_throws BoundsError deleteat!([], [2])
  @test deleteat!([], []) == []
  @test deleteat!([], Bool[]) == []
+ let a = Vector{Any}(undef, 2)
+ a[1] = 1
+ @test isassigned(deleteat!(copy(a), [2]), 1)
+ @test !isassigned(deleteat!(copy(a), [1]), 1)
+ end
 end
 
 @testset "comprehensions" begin