Fix stack(; dims) on containers with HasLength eltype & HasShape elements (#56777)

Fixes #56771

While fixing this I saw #56776, will comment there on differences.

I think the reason this line existed was probably to handle `Tuple`, for
which `Base.IteratorSize(Tuple) == Base.HasLength()` although they
behave like `HasShape{1}`. I'd like to check a little more for
unintended consequences, hence mark this draft for now.

Co-authored-by: Andy Dienes <[email protected]>

Author: mcabbott

base/abstractarray.jl

@@ -2939,8 +2939,6 @@ _iterator_axes(x, ::IteratorSize) = axes(x)
 # For some dims values, stack(A; dims) == stack(vec(A)), and the : path will be faster
 _typed_stack(dims::Integer, ::Type{T}, ::Type{S}, A) where {T,S} =
     _typed_stack(dims, T, S, IteratorSize(S), A)
-_typed_stack(dims::Integer, ::Type{T}, ::Type{S}, ::HasLength, A) where {T,S} =
-    _typed_stack(dims, T, S, HasShape{1}(), A)
 function _typed_stack(dims::Integer, ::Type{T}, ::Type{S}, ::HasShape{N}, A) where {T,S,N}
     if dims == N+1
         _typed_stack(:, T, S, A, (_vec_axis(A),))

test/abstractarray.jl

@@ -1880,6 +1880,33 @@ end
     end
 end
 
+@testset "issue 56771, stack(; dims) on containers with HasLength eltype & HasShape elements" begin
+    for T in (Matrix, Array, Any)
+        xs = T[rand(2,3) for _ in 1:4]
+        @test size(stack(xs; dims=1)) == (4,2,3)
+        @test size(stack(xs; dims=2)) == (2,4,3)  # this was the problem case, for T=Array
+        @test size(stack(xs; dims=3)) == (2,3,4)
+        @test size(stack(identity, xs; dims=2)) == (2,4,3)
+        @test size(stack(x for x in xs if true; dims=2)) == (2,4,3)
+
+        xmat = T[rand(2,3) for _ in 1:4, _ in 1:5]
+        @test size(stack(xmat; dims=1)) == (20,2,3)
+        @test size(stack(xmat; dims=2)) == (2,20,3)
+        @test size(stack(xmat; dims=3)) == (2,3,20)
+    end
+
+    it = Iterators.product(1:2, 3:5)
+    @test size(it) == (2,3)
+    @test Base.IteratorSize(typeof(it)) == Base.HasShape{2}()
+    @test Base.IteratorSize(Iterators.ProductIterator) == Base.HasLength()
+    for T in (typeof(it), Iterators.ProductIterator, Any)
+        ys = T[it for _ in 1:4]
+        @test size(stack(ys; dims=2)) == (2,4,3)
+        @test size(stack(identity, ys; dims=2)) == (2,4,3)
+        @test size(stack(y for y in ys if true; dims=2)) == (2,4,3)
+    end
+end
+
 @testset "keepat!" begin
     a = [1:6;]
     @test a === keepat!(a, 1:5)