Add Channel() constr, defaults to unbufferred channel

Add unit tests for default constructor

Author: NHDaly

base/channels.jl

@@ -8,7 +8,7 @@ Representation of a channel passing objects of type `T`.
 abstract type AbstractChannel{T} end
 
 """
-    Channel{T}(sz::Int)
+    Channel{T}(sz::Int=0)
 
 Constructs a `Channel` with an internal buffer that can hold a maximum of `sz` objects
 of type `T`.
@@ -19,8 +19,12 @@ And vice-versa.
 
 Other constructors:
 
+* `Channel()`: default constructor, equivalent to `Channel{Any}(0)`
 * `Channel(Inf)`: equivalent to `Channel{Any}(typemax(Int))`
 * `Channel(sz)`: equivalent to `Channel{Any}(sz)`
+
+!!! compat "Julia 1.3"
+  The default constructor `Channel()` was added in Julia 1.3.
 """
 mutable struct Channel{T} <: AbstractChannel{T}
     cond_take::Threads.Condition                 # waiting for data to become available
@@ -48,6 +52,7 @@ function Channel{T}(sz::Float64) where T
     return Channel{T}(sz)
 end
 Channel(sz) = Channel{Any}(sz)
+Channel() = Channel{Any}(0)
 
 # special constructors
 """

test/channels.jl

@@ -13,6 +13,9 @@ using Random
 end
 
 @testset "various constructors" begin
+    c = Channel()
+    @test eltype(c) == Any
+
     c = Channel(1)
     @test eltype(c) == Any
     @test put!(c, 1) == 1
@@ -31,7 +34,6 @@ end
     tvals = Int[take!(c) for i in 1:10^6]
     @test pvals == tvals
 
-    @test_throws MethodError Channel()
     @test_throws ArgumentError Channel(-1)
     @test_throws InexactError Channel(1.5)
 end
@@ -48,6 +50,10 @@ end
     @test c.sz_max == 0
     @test collect(c) == 1:100
 
+    c = Channel() do c; put!(1); put!("hi") end
+    @test c.sz_max == 0
+    @test collect(c) == [1, "hi"]
+
     c = Channel{Int}(Inf) do c; put!(c,1); end
     @test eltype(c) == Int
     @test c.sz_max == typemax(Int)