Check task state on iterate

Before, iterate on Channel would return `nothing` when the channel was closed,
not checking if it was closed with an error.

base/channels.jl

@@ -676,6 +676,15 @@ function iterate(c::Channel, state=nothing)
             end
         end
     else
+        # If the channel was closed with an exception, it needs to be thrown
+        if (@atomic :acquire c.state) === :closed
+            e = c.excp
+            if isa(e, InvalidStateException) && e.state === :closed
+                nothing
+            else
+                throw(e)
+            end
+        end
         return nothing
     end
 end

test/channels.jl

@@ -499,12 +499,35 @@ end
     end
 end
 
+struct CustomError <: Exception end
+
 @testset "check_channel_state" begin
     c = Channel(1)
     close(c)
     @test !isopen(c)
     c.excp === nothing # to trigger the branch
     @test_throws InvalidStateException Base.check_channel_state(c)
+
+    # Issue 52974 - closed channels with exceptions
+    # must be thrown on iteration, if channel is empty
+    c = Channel(2)
+    put!(c, 5)
+    close(c, CustomError())
+    @test take!(c) == 5
+    @test_throws CustomError iterate(c)
+
+    c = Channel(Inf)
+    put!(c, 1)
+    close(c)
+    @test take!(c) == 1
+    @test_throws InvalidStateException take!(c)
+    @test_throws InvalidStateException put!(c, 5)
+
+    c = Channel(3)
+    put!(c, 1)
+    close(c)
+    @test first(iterate(c)) == 1
+    @test isnothing(iterate(c))
 end
 
 # PR #36641