cmd/gofmt: comment in return params is misplaced

[benesch]

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

go version go1.11 darwin/amd64

Does this issue reproduce with the latest release?

Yes.

What did you do?

Ran gofmt on this program (https://play.golang.org/p/1rASHggr4wx):

package foo

func bar() (
	// This return value is extremely subtle! Please make sure you understand
	// what it does.
	string,
) {
	return "42"
}

What did you expect to see?

The input, unchanged.

What did you see instead?

The program reformatted with the comment in a very strange place (https://play.golang.org/p/NujRVxVN6N5):

package foo

func bar() string {// This return value is extremely subtle! Please make sure you understand
	// what it does.

	return "42"
}

Arguably not that common to stick a comment there, but I find the resulting code extremely surprising. Note that this nearly identical program works just fine (https://play.golang.org/p/Hpfcc5HX00l) because the presence of bazzle makes the parens required and so gofmt doesn't attempt a reformatting:

package foo

func bar() (
	// This return value is extremely subtle! Please make sure you understand
	// what it does.
	bazzle string,
) {
	return "42"
}

/cc @griesemer since you seem to get pinged on all the gofmt issues eventually

[agnivade]

Most probably related to #22631.

[griesemer]

Seems to be a combination of problems here. I don't think this is related to #22631, this seems more an issue of not having all position information in the AST, and the fact that we don't print unnecessary ()'s around single, unnamed results.

Should be fixed at some point but there's no urgency. Also, independently, not a great place to put a comment in the first place (but that's just my personal taste).

[bcmills]

Dup of #23040?

[benesch]

Dup of #23040?

💯 thanks! (I did try to find an existing issue before I filed, but issues like this are more or less impossible to search for.)

[griesemer]

This is not a duplicate of #23040 - that one is about ast.CommentMaps. There are no comment maps involved here. Reopening.