What will be the result of writing to a closed channel? (deadlock / panic / operation returns an error)

Writing to a closed (non-open for writing) channel in Go causes a panic.

If the channel is closed, attempting to send a value to it results in a runtime panic with the message "send on closed channel".

Example:

ch := make(chan int)
close(ch)
ch <- 1 // panic: send on closed channel

Thus, the operation does not return an error and does not block — a immediate panic occurs. To avoid this, it is common to check whether the channel is closed before sending, or to use recover to handle the panic.