Not limited to function parameters:

package main

func main() {
	i := 42
	(i++)
}

$ go run prova.go
# command-line-arguments
./prova.go:5: syntax error: unexpected ++, expecting )
./prova.go:6: syntax error: non-declaration statement outside function body

Also

package main

func main() {
	i := 42
	(var int)
}

gives the same double error. Seems like the parser is confused by certain syntax issues when inside _{L,R}paren.

Except for grouping of declarations (icl. parameter declarations), inside ()'s we can only have expressions. Possibly error recovery could be a bit stronger in that case.

It's tricky to come up with a good parser recovery strategy in cases like these. The culprit is the syntax.parser.want method:

 func (p *parser) want(tok token) {
        if !p.got(tok) {
                p.syntax_error("expecting " + tok.String())
                p.advance()
        }
 }

If a ++ (or any other token) is found instead of the closing token with tok == ), an error is reported and the offending token is consumed. This assumes that the expected token is simply missing. In these cases, the expected ) follows. So a simple way to work around this here is to change the want method as follows:

@@ -90,7 +90,8 @@ func (p *parser) got(tok token) bool {
 func (p *parser) want(tok token) {
        if !p.got(tok) {
                p.syntax_error("expecting " + tok.String())
-               p.advance()
+               p.advance(tok)
+               p.got(tok) // try once more
        }
 }

This will fix these issues, but cause others (run the tests).

Leaving open for further exploration. Not urgent.

This appears to be fixed in c37090f.

Confirming. Thanks for noticing this, @0xmohit .