cmd/compile: init info of OAS node in a select case is being dropped

[danscales]

Issues exists in current master (go1.18-devel), but has probably existed for a while.

When an OAS node is converted to an OSELRECV2 node in tcSelect(), any DCL node in the Init field (due to a := assignment) is being dropped, since a completely new node is being created and the Init field was not set.

Thanks for the test case created by @mdempsky:

package main

func main() {
	ch := make(chan int, 1)

	var ptrs [2]*int
	for i := range ptrs {
		ch <- i
		select {
		case x := <-ch:
			ptrs[i] = &x
		}
	}

	for i, ptr := range ptrs {
		if *ptr != i {
			println("FAIL:", *ptr, "!=", i)
		}
	}
}

[gopherbot]

Change https://golang.org/cl/348569 mentions this issue: cmd/compile: fix case where init info of OAS node is dropped

[rhysh]

The problem this fixes was introduced by 0328c3b. It affects the Go 1.17 series, including go1.17.3.

$ ./bin/go version && ./bin/go run ~/issue48289.go
go version go1.16.10 linux/amd64

$ ./bin/go version && ./bin/go run ~/issue48289.go
go version go1.17.3 linux/amd64
FAIL: 1 != 0

$ ./bin/go version && ./bin/go run ~/issue48289.go
go version devel +88e1415d08 Thu Dec 17 04:43:59 2020 +0000 linux/amd64

$ ./bin/go version && ./bin/go run ~/issue48289.go
go version devel +0328c3b660 Thu Dec 17 15:50:38 2020 +0000 linux/amd64
FAIL: 1 != 0

@gopherbot please consider this for backport to 1.17

[gopherbot]

Backport issue(s) opened: #49510 (for 1.16), #49511 (for 1.17).

Remember to create the cherry-pick CL(s) as soon as the patch is submitted to master, according to https://golang.org/wiki/MinorReleases.