What did you do?

https://play.golang.org/p/ORxhPRy1cl

What did you expect to see?

7
42

Since ProduceTo declares its argument as a chan<- int, it can only ever write ints (or close) to the channel. ints can be assigned to variables of type interface{}, and ints can be sent to a chan interface{}, so it'd be nice to let ProduceTo write to a chan interface{} even though its argument is chan<- int.

Similarly, since ConsumeFrom takes a <-chan interface{}, so it must be able to handle any interface{} value read from the channel. By passing a chan int value, the programmer knows that in reality ConsumeFrom will only receive int values, but ConsumeFrom is ready. ConsumeFrom can handle anything! Believe in ConsumeFrom!

What did you see instead?

tmp/sandbox755940094/main.go:17: cannot use c1 (type chan interface {}) as type chan<- int in argument to ProduceTo
tmp/sandbox755940094/main.go:22: cannot use c2 (type chan int) as type <-chan interface {} in argument to ConsumeFrom

Working around the non-assignability of appropriately covariant / contravariant channels usually needs another channel and goroutine: https://play.golang.org/p/b2JsBdvLy1.