Consider the following program:

package main

func main() {
	type Channel chan int
	type SendChannel chan<- int
	type ReceiveChannel <-chan int

	var channel Channel
	var send SendChannel
	var receive ReceiveChannel
	_, _, _ = channel, send, receive

	// Not allowed:
	send = SendChannel(channel)       // cannot convert channel (variable of type Channel) to SendChannel
	receive = ReceiveChannel(channel) // cannot convert channel (variable of type Channel) to ReceiveChannel

	// Allowed:
	send = SendChannel((chan<- int)(channel))
	receive = ReceiveChannel((<-chan int)(channel))
}

As demonstrated, it is not allowed to convert a defined bidirectional channel type to a defined unidirectional channel type. This is caused by the following condition in the Go spec:

Assignability

...

• x is a bidirectional channel value, T is a channel type, x's type V and T have identical element types, and at least one of V or T is not a defined type.

However, it is allowed to first convert to a non-defined type, then to the defined type. If I can convert a defined type V to an intermediate non-defined type, then to a defined type T, then I should be able to convert directly from V to T.

To clarify, the proposal is not to change assignability, but to add a conversion case that allows the conversion of a defined bidirectional channel type to a defined unidirectional channel type, where the element types are identical.