deprecated Channel() instead of throwing an error

base/channels.jl

@@ -26,10 +26,21 @@ type Channel{T} <: AbstractChannel
         end
         new(Condition(), Condition(), :open, Array{T}(0), sz, Array{Condition}(0))
     end
+
+    # deprecated empty constructor
+    function Channel()
+        depwarn(string("The empty constructor Channel() is deprecated. ",
+                        "The channel size needs to be specified explictly. ",
+                        "Defaulting to Channel{$T}(32)."), :Channel)
+        Channel(32)
+    end
 end
 
 Channel(sz) = Channel{Any}(sz)
 
+# deprecated empty constructor
+Channel() = Channel{Any}()
+
 closed_exception() = InvalidStateException("Channel is closed.", :closed)
 
 isbuffered(c::Channel) = c.sz_max==0 ? false : true

base/deprecated.jl

@@ -1023,4 +1023,7 @@ end))
 
 @deprecate is (===)
 
+# NOTE: Deprecation of Channel{T}() is implemented in channels.jl.
+# To be removed from there when 0.6 deprecations are removed.
+
 # End deprecations scheduled for 0.6

test/channels.jl

@@ -20,7 +20,9 @@ pvals = map(i->put!(c,i), 1:10^6)
 tvals = Int[take!(c) for i in 1:10^6]
 @test pvals == tvals
 
-@test_throws MethodError Channel()
+# Uncomment line below once deprecation support has been removed.
+# @test_throws MethodError Channel()
+
 @test_throws ArgumentError Channel(-1)
 @test_throws InexactError Channel(1.5)