add Iterators.countfrom(::Any, ::Any)

NEWS.md

@@ -51,6 +51,7 @@ Standard library changes
   constructing the range. ([#40382])
 * TCP socket objects now expose `closewrite` functionality and support half-open mode usage ([#40783]).
 * Intersect returns a result with the eltype of the type-promoted eltypes of the two inputs ([#41769]).
+* `Iterators.countfrom` now accepts any type that defines `+`. ([#37747])
 
 #### InteractiveUtils
 * A new macro `@time_imports` for reporting any time spent importing packages and their dependencies ([#41612])

base/iterators.jl

@@ -592,8 +592,8 @@ IteratorSize(::Type{<:Rest{I}}) where {I} = rest_iteratorsize(IteratorSize(I))
 
 # Count -- infinite counting
 
-struct Count{S<:Number}
-    start::S
+struct Count{T,S}
+    start::T
     step::S
 end
 
@@ -613,11 +613,13 @@ julia> for v in Iterators.countfrom(5, 2)
 9
 ```
 """
-countfrom(start::Number, step::Number) = Count(promote(start, step)...)
-countfrom(start::Number)               = Count(start, oneunit(start))
-countfrom()                            = Count(1, 1)
+countfrom(start::T, step::S) where {T,S} = Count{typeof(start+step),S}(start, step)
+countfrom(start::Number, step::Number)   = Count(promote(start, step)...)
+countfrom(start)                         = Count(start, oneunit(start))
+countfrom()                              = Count(1, 1)
 
-eltype(::Type{Count{S}}) where {S} = S
+
+eltype(::Type{Count{T,S}}) where {T,S} = T
 
 iterate(it::Count, state=it.start) = (state, state + it.step)
 

test/iterators.jl

@@ -3,6 +3,7 @@
 using Base.Iterators
 using Random
 using Base: IdentityUnitRange
+using Dates: Date, Day
 
 @test Base.IteratorSize(Any) isa Base.SizeUnknown
 
@@ -123,7 +124,7 @@ end
 
 # countfrom
 # ---------
-let i = 0, k = 1
+let i = 0, k = 1, l = 0
     for j = countfrom(0, 2)
         @test j == i*2
         i += 1
@@ -134,6 +135,15 @@ let i = 0, k = 1
         k += 1
         k <= 10 || break
     end
+    # test that `start` promotes to `typeof(start+step)`
+    for j = countfrom(Int[0, 0], Float64[1.0, 2.0])
+        @test j isa Vector{Float64}
+        @test j == l*[1, 2]
+        l += 1
+        l <= 10 || break
+    end
+    # test with `start` and `step` having different types
+    @test collect(take(countfrom(Date(2020,12,25), Day(1)), 12)) == range(Date(2020,12,25), step=Day(1), length=12)
 end
 
 # take