From 4ae91695b3964c6974e34988ac8e4a7d197ad418 Mon Sep 17 00:00:00 2001
From: Andy Ferris <ferris.andy@gmail.com>
Date: Wed, 20 May 2020 11:36:54 +1000
Subject: [PATCH] Add a default implementation of `length` using `iterate`.

---
 NEWS.md           |  1 +
 base/generator.jl | 12 ++++++++++++
 test/missing.jl   |  1 +
 3 files changed, 14 insertions(+)

diff --git a/NEWS.md b/NEWS.md
index ea03e90990f59..9de2d9a6a63e2 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -43,6 +43,7 @@ Standard library changes
 ------------------------
 * The `nextprod` function now accepts tuples and other array types for its first argument ([#35791]).
 * The function `isapprox(x,y)` now accepts the `norm` keyword argument also for numeric (i.e., non-array) arguments `x` and `y` ([#35883]).
+* There is a default implementation of `length` for all iterables with `IteratorSize` of `SizeUnknown` (using simple iteration) ([#35947]).
 
 #### LinearAlgebra
 
diff --git a/base/generator.jl b/base/generator.jl
index 7cd075d255b01..def286527a194 100644
--- a/base/generator.jl
+++ b/base/generator.jl
@@ -97,6 +97,18 @@ IteratorSize(::Type{Any}) = SizeUnknown()
 
 haslength(iter) = IteratorSize(iter) isa Union{HasShape, HasLength}
 
+function length(iter)
+    if IteratorSize(iter) isa SizeUnknown
+        i = 0
+        for _ in iter
+            i += 1
+        end
+        return i
+    else
+        throw(MethodError(length, (iter,)))
+    end
+end
+
 abstract type IteratorEltype end
 struct EltypeUnknown <: IteratorEltype end
 struct HasEltype <: IteratorEltype end
diff --git a/test/missing.jl b/test/missing.jl
index 850482585b2cd..e84548b9d921f 100644
--- a/test/missing.jl
+++ b/test/missing.jl
@@ -363,6 +363,7 @@ end
 @testset "skipmissing" begin
     x = skipmissing([1, 2, missing, 4])
     @test eltype(x) === Int
+    @test length(x) == 3
     @test collect(x) == [1, 2, 4]
     @test collect(x) isa Vector{Int}