I’m confused. Your expected output isn’t sufficient (nothing after “found”).

Are you saying that it should not say a.go:11:37: missing ',' in argument list at all but the first message (a.go:6:4: expected operand, found ',') is OK?

yes.
I modified the expected output.
Looks the comment block doesn't matter for the redundant error message.

This is another case of go/parser being behind cmd/compile/internal/syntax in terms of reporting errors to humans.

$ gofmt f4.go
f4.go:6:4: expected operand, found ','
f4.go:7:3: missing ',' in argument list
$ go build f4.go
# command-line-arguments
./f4.go:6:4: syntax error: unexpected comma, expecting expression

I'll let @griesemer decide what to do.

Marking as unplanned. go/parser is likely falling behind the compiler due to the compiler's parser (package syntax) being actively improved whenever we run into unsatisfying error messages. It's fairly time-consuming to back-port those fixes because the parsers don't have the same structure, and small local changes sometimes have unexpected consequences with respect to error handling (hence those changes, even if small, are time-consuming to get right).

Ideally, in the long run we should migrate to package syntax. Failing that, one option might be to replace go/parser with the syntax package parser while keeping the same API. That might be not too hard since the syntax trees are reasonably close.

Change https://golang.org/cl/308612 mentions this issue: go/parser: improve error recovery in parseOperand