cmd/compile: declared but not used if use type switch on type parameter value

[XiaodongLoong]

What version of Go are you using (go version)?

$ go version
go version devel go1.18-3b5eec9370 Fri Jan 14 21:54:39 2022 +0000 darwin/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

go env Output

$ go env

What did you do?

package p

func f[T any](a T) {
	x := 1
	z := 3
	switch a.(type) {
	case int:
		println(x)
	}
	println(z)
}

$ bin/go tool compile t2.go 
t2.go:4:2: x declared but not used
t2.go:6:9: cannot use type switch on type parameter value a (variable of type T constrained by any)

What did you expect to see?

Don't output "x declared but not used"

What did you see instead?

It should only output as following :
t2.go:6:9: cannot use type switch on type parameter value a (variable of type T constrained by any)

[gopherbot]

Change https://golang.org/cl/378834 mentions this issue: cmd/compile/internal/types2: avoid declared but not used error if type switch stmt failed

[ianlancetaylor]

Thanks. Closing as dup of #50493.