type stability

base/abstractarray.jl

@@ -2611,15 +2611,20 @@ end
 
 function _vstack_plus(itr)
     z = iterate(itr)
-    isnothing(z) && throw(ArgumentError("cannot stack an empty collection"))
+    z === nothing && throw(ArgumentError("cannot stack an empty collection"))
     val, state = z
     val isa Union{AbstractArray, Tuple} || throw(ArgumentError("cannot stack elements of type $(typeof(val))"))
 
     axe = axes(val)
     len = length(val)
     n = haslength(itr) ? len*length(itr) : nothing
 
-    v = similar(val isa Tuple ? (1:0) : val, eltype(val), something(n, len))
+    v = if val isa Tuple
+        T = mapreduce(typeof, promote_type, val)
+        similar(1:0, T, something(n, len))
+    else
+        similar(val, something(n, len))
+    end
     copyto!(v, 1, val, firstindex(val), len)
 
     w = _stack_rest!(v, 0, n, axe, itr, state)
@@ -2630,12 +2635,16 @@ function _stack_rest!(v::AbstractVector, i, n, axe, itr, state)
     len = prod(length, axe; init=1)
     while true
         z = iterate(itr, state)
-        isnothing(z) && return v
+        z === nothing && return v
         val, state = z
         axes(val) == axe || throw(DimensionMismatch(
             "expected a consistent size, got axes $(UnitRange.(axes(val))) compared to $(UnitRange.(axe)) for the first"))
         i += 1
-        T′ = promote_type(eltype(v), eltype(val))
+        T′ = if val isa Tuple
+            promote_type(eltype(v), mapreduce(typeof, promote_type, val))
+        else
+            promote_type(eltype(v), eltype(val))
+        end
         if T′ <: eltype(v)
             if n isa Integer
                 copyto!(v, i*len+1, val, firstindex(val), len)

test/abstractarray.jl

@@ -1565,6 +1565,11 @@ end
         X3 = stack(x for x in args if true)
         @test X3 == Y
         @test typeof(X3) === typeof(Y)
+
+        if isconcretetype(eltype(args))
+            @inferred stack(args)
+            @inferred stack(x for x in args)
+        end
     end
     # Higher dims
     @test size(stack([rand(2,3) for _ in 1:4, _ in 1:5])) == (2,3,4,5)
@@ -1574,12 +1579,13 @@ end
     # Tuples
     @test stack([(1,2), (3,4)]) == [1 3; 2 4]
     @test stack(((1,2), (3,4))) == [1 3; 2 4]
-    @test size(stack(Iterators.product(1:3, 1:4))) == (2,3,4)
-    @test stack([('a', 'b'), ('c', 'd')]) == ['a' 'c'; 'b' 'd']
+    @test size(@inferred stack(Iterators.product(1:3, 1:4))) == (2,3,4)
+    @test @inferred(stack([('a', 'b'), ('c', 'd')])) == ['a' 'c'; 'b' 'd']
+    @test @inferred(stack([(1,2+3im), (4, 5+6im)])) isa Matrix{Complex{Int}}
 
     # stack(f, iter)
-    @test stack(x -> [x, 2x], 3:5) == [3 4 5; 6 8 10]
-    @test stack(x -> x*x'/2, [1:2, 3:4]) == [0.5 1.0; 1.0 2.0;;; 4.5 6.0; 6.0 8.0]
+    @test @inferred(stack(x -> [x, 2x], 3:5)) == [3 4 5; 6 8 10]
+    @test @inferred(stack(x -> x*x'/2, [1:2, 3:4])) == [0.5 1.0; 1.0 2.0;;; 4.5 6.0; 6.0 8.0]
 
     # Mismatched sizes
     @test_throws DimensionMismatch stack([1:2, 1:3])