cmd/compile: give better error for composite literal parsing ambiguity

[empijei]

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

$ go version
go version go1.12 linux/amd64

Does this issue reproduce with the latest release?

Yes

What did you do?

type foo struct{}

func main() {
	bar := foo{}
	if bar == foo{} {
		fmt.Println("true")
		return
	}
	fmt.Println("false")
}

Play link

What did you expect to see?

true

What did you see instead?

prog.go:11:18: syntax error: unexpected { at end of statement
prog.go:15:2: syntax error: non-declaration statement outside function body

Please note that adding parentheses works

	if bar == (foo{}) {

Is this intended? If so, shouldn't the compiler give better errors?

[ianlancetaylor]

Search for "parsing ambiguity" in https://golang.org/ref/spec#Composite_literals .

I don't know whether the compiler could give a better error message. I'll retitle the issue for that.

[arashbina]

Do you still need someone to investigate this?

[ianlancetaylor]

The thing to investigate would be whether it is straightforward for the compiler to give a better error message for this case. If you want to look into it, go ahead, but please be aware that this is a subtle issue that will need to consider several different kinds of incorrect code to make sure that the error message does not become more misleading. Thanks.

[arashbina]

Ok thanks. I'll take a look and update here.

[quasilyte]

I think the main complexity here is that the error message probably originates from the parser, and we don't know about types at that point, so I think it's impossible for a parser in the current state to tell whether foo is a type or not. It's relevant not only to the Go internal parser but also to go/parser.

[empijei]

I think this is rather complicated and would probably come up to be a hack. I looked into it and IMO it would add too much complexity to address it. I don't want that and since this is already documented I guess it can stay as it is.

Closing as won't fix. Feel free to re-open if you can come up with a good and simple solution.