go/parser: fix "zero day" parse error

(a b string, ok bool) is not a valid signature

Fixes #8656.

LGTM=adonovan
R=adonovan
CC=golang-codereviews
https://golang.org/cl/137140043

src/pkg/go/parser/parser.go

@@ -823,9 +823,10 @@ func (p *parser) parseParameterList(scope *ast.Scope, ellipsisOk bool) (params [
 		// parameter or result variable is the function body.
 		p.declare(field, nil, scope, ast.Var, idents...)
 		p.resolve(typ)
-		if p.tok == token.COMMA {
-			p.next()
+		if !p.atComma("parameter list") {
+			return
 		}
+		p.next()
 		for p.tok != token.RPAREN && p.tok != token.EOF {
 			idents := p.parseIdentList()
 			typ := p.parseVarType(ellipsisOk)
@@ -840,15 +841,15 @@ func (p *parser) parseParameterList(scope *ast.Scope, ellipsisOk bool) (params [
 			}
 			p.next()
 		}
-	} else {
-		// Type { "," Type } (anonymous parameters)
-		params = make([]*ast.Field, len(list))
-		for i, typ := range list {
-			p.resolve(typ)
-			params[i] = &ast.Field{Type: typ}
-		}
+		return
 	}
 
+	// Type { "," Type } (anonymous parameters)
+	params = make([]*ast.Field, len(list))
+	for i, typ := range list {
+		p.resolve(typ)
+		params[i] = &ast.Field{Type: typ}
+	}
 	return
 }
 

src/pkg/go/parser/short_test.go

@@ -93,6 +93,7 @@ var invalids = []string{
 	`package p; func f() { go f /* ERROR HERE "function must be invoked" */ }`,
 	`package p; func f() { defer func() {} /* ERROR HERE "function must be invoked" */ }`,
 	`package p; func f() { go func() { func() { f(x func /* ERROR "expected '\)'" */ (){}) } } }`,
+	`package p; func f() (a b string /* ERROR "expected '\)'" */ , ok bool) // issue 8656`,
 }
 
 func TestInvalid(t *testing.T) {