Fix collect on stateful generator (#41919)

Previously this code would drop 1 from the length of some generators.

Fixes #35530

Author: jakobnissen

base/array.jl

@@ -758,21 +758,25 @@ else
     end
 end
 
-_array_for(::Type{T}, itr, ::HasLength) where {T} = Vector{T}(undef, Int(length(itr)::Integer))
-_array_for(::Type{T}, itr, ::HasShape{N}) where {T,N} = similar(Array{T,N}, axes(itr))
+_array_for(::Type{T}, itr, isz::HasLength) where {T} = _array_for(T, itr, isz, length(itr))
+_array_for(::Type{T}, itr, isz::HasShape{N}) where {T,N} = _array_for(T, itr, isz, axes(itr))
+_array_for(::Type{T}, itr, ::HasLength, len) where {T} = Vector{T}(undef, len)
+_array_for(::Type{T}, itr, ::HasShape{N}, axs) where {T,N} = similar(Array{T,N}, axs)
 
 function collect(itr::Generator)
     isz = IteratorSize(itr.iter)
     et = @default_eltype(itr)
     if isa(isz, SizeUnknown)
         return grow_to!(Vector{et}(), itr)
     else
+        shape = isz isa HasLength ? length(itr) : axes(itr)
         y = iterate(itr)
         if y === nothing
             return _array_for(et, itr.iter, isz)
         end
         v1, st = y
-        collect_to_with_first!(_array_for(typeof(v1), itr.iter, isz), v1, itr, st)
+        arr = _array_for(typeof(v1), itr.iter, isz, shape)
+        return collect_to_with_first!(arr, v1, itr, st)
     end
 end
 

test/iterators.jl

@@ -292,6 +292,15 @@ let (a, b) = (1:3, [4 6;
     end
 end
 
+# collect stateful iterator
+let
+    itr = (i+1 for i in Base.Stateful([1,2,3]))
+    @test collect(itr) == [2, 3, 4]
+    A = zeros(Int, 0, 0)
+    itr = (i-1 for i in Base.Stateful(A))
+    @test collect(itr) == Int[] # Stateful do not preserve shape
+end
+
 # with 1D inputs
 let a = 1:2,
     b = 1.0:10.0,
@@ -860,4 +869,4 @@ end
     @test Iterators.peel(1:10)[2] |> collect == 2:10
     @test Iterators.peel(x^2 for x in 2:4)[1] == 4
     @test Iterators.peel(x^2 for x in 2:4)[2] |> collect == [9, 16]
-end
+end