cmd/compile: misleading error message for index expression

[mdevan]

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

$ go version
go version go1.17.2 linux/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?

In this code:

	if a, b := c["foo"]; b {
		fmt.Print(a)
	}

if the variable 'c' is not defined, these errors are reported:

./prog.go:8:10: assignment mismatch: 2 variables but 1 value
./prog.go:8:13: undefined: c

The first error message is misleading and redundant.

https://play.golang.org/p/I0TcR8834tK

What did you expect to see?

Only the second error message (undefined: c).

What did you see instead?

As shown above.

[odeke-em]

Thank you @mdevan for reporting this issue! It is fixed for Go1.18 thanks be to the excellent work that @findleyr and @griesemer implemented this cycle.

$ go version && go run main.go 
go version devel go1.18-5344dcae41 Tue Nov 9 00:08:42 2021 +0000 darwin/amd64
# command-line-arguments
./main.go:8:13: undefined: c

You can experiment with the latest Go version by following https://pkg.go.dev/golang.org/dl/gotip#pkg-overview

$ go install golang.org/dl/gotip@latest
$ gotip download

I shall therefore close this issue, but please don't hesitate to reach out and file more bugs as you see them.