make cumsum result type consistent with sum

NEWS.md

@@ -79,7 +79,7 @@ Library improvements
   * Arithmetic is type-preserving for more types; e.g. `(x::Int8) + (y::Int8)` now
     yields an `Int8` ([#3759]).
 
-  * Reductions (e.g. `reduce`, `sum`) widen small types (integers smaller than `Int`, and `Float16`).
+  * Reductions (e.g. `reduce`, `sum`) widen small types (integers smaller than `Int`, and `Float16`).  Similarly for `cumsum` ([#9665]).
 
   * New `Dates` module for calendar dates and other time-interval calculations ([#7654]).
 
@@ -1149,5 +1149,8 @@ Too numerous to mention.
 [#9261]: https://github.com/JuliaLang/julia/issues/9261
 [#9271]: https://github.com/JuliaLang/julia/issues/9271
 [#9294]: https://github.com/JuliaLang/julia/issues/9294
-[#9569]: https://github.com/JuliaLang/julia/issues/9569
+[#9418]: https://github.com/JuliaLang/julia/issues/9418
 [#9452]: https://github.com/JuliaLang/julia/issues/9452
+[#9569]: https://github.com/JuliaLang/julia/issues/9569
+[#9578]: https://github.com/JuliaLang/julia/issues/9578
+[#9665]: https://github.com/JuliaLang/julia/issues/9665

base/array.jl

@@ -1452,11 +1452,11 @@ symdiff(a) = a
 symdiff(a, b) = union(setdiff(a,b), setdiff(b,a))
 symdiff(a, b, rest...) = symdiff(a, symdiff(b, rest...))
 
-_cumsum_type{T<:Number}(v::AbstractArray{T}) = typeof(+zero(T))
-_cumsum_type(v) = typeof(v[1]+v[1])
+_cumsum_type{T<:Number}(v::AbstractArray{T}) = typeof(r_promote(AddFun, zero(T)::T))
+_cumsum_type(v) = typeof(r_promote(AddFun, v[1]))
 
-for (f, fp, op) = ((:cumsum, :cumsum_pairwise!, :+),
-                   (:cumprod, :cumprod_pairwise!, :*) )
+for (f, f!, fp, op) = ((:cumsum, :cumsum!, :cumsum_pairwise!, :+),
+                       (:cumprod, :cumprod!, :cumprod_pairwise!, :*) )
     # in-place cumsum of c = s+v[range(i1,n)], using pairwise summation
     @eval function ($fp){T}(v::AbstractVector, c::AbstractVector{T}, s, i1, n)
         local s_::T # for sum(v[range(i1,n)]), i.e. sum without s
@@ -1475,16 +1475,18 @@ for (f, fp, op) = ((:cumsum, :cumsum_pairwise!, :+),
         return s_
     end
 
-    @eval function ($f)(v::AbstractVector)
+    @eval function ($f!)(result::AbstractVector, v::AbstractVector)
         n = length(v)
+        if n == 0; return result; end
+        ($fp)(v, result, $(op==:+ ? :(zero(v[1])) : :(one(v[1]))), 1, n)
+        return result
+    end
+
+    @eval function ($f)(v::AbstractVector)
         c = $(op===:+ ? (:(similar(v,_cumsum_type(v)))) :
                         (:(similar(v))))
-        if n == 0; return c; end
-        ($fp)(v, c, $(op==:+ ? :(zero(v[1])) : :(one(v[1]))), 1, n)
-        return c
+        return ($f!)(c, v)
     end
-
-    @eval ($f)(A::AbstractArray) = ($f)(A, 1)
 end
 
 for (f, op) = ((:cummin, :min), (:cummax, :max))

test/arrayops.jl

@@ -978,4 +978,11 @@ a = [ [ 1 0 0 ], [ 0 0 0 ] ]
 # issue #9648
 let x = fill(1.5f0, 10^7)
     @test abs(1.5f7 - cumsum(x)[end]) < 3*eps(1.5f7)
+    @test cumsum(x) == cumsum!(similar(x), x)
+end
+
+# cumsum type consistency (discussed in #9650)
+let x = Uint8[1,2,3,4,6,7]
+    @test eltype(cumsum(x)) == typeof(sum(x)) == eltype(cumsum(reshape(x,3,2)))
+    @test cumsum(x) == cumsum!(similar(x),x) == cumsum!(similar(x,Int), x)
 end