fix iterate(@thunk ()) (#534)

Author: mzgubic

Project.toml

@@ -1,6 +1,6 @@
 name = "ChainRulesCore"
 uuid = "d360d2e6-b24c-11e9-a2a3-2a2ae2dbcce4"
-version = "1.11.4"
+version = "1.11.5"
 
 [deps]
 Compat = "34da2185-b29b-5c13-b0c7-acf172513d20"

src/tangent_types/thunks.jl

@@ -11,7 +11,9 @@ Base.Broadcast.broadcastable(x::AbstractThunk) = broadcastable(unthunk(x))
 
 @inline function Base.iterate(x::AbstractThunk)
     val = unthunk(x)
-    element, state = iterate(val)
+    el_and_state = iterate(val)
+    el_and_state isa Nothing && return nothing
+    element, state = el_and_state
     return element, (val, state)
 end
 

test/tangent_types/thunks.jl

@@ -15,6 +15,8 @@
         for (i, j) in zip(a, t)
             @test i == j
         end
+
+        @test nothing === iterate(@thunk ()) == iterate(())
     end
 
     @testset "show" begin