go/parser: require that '...' parameters are followed by a type

(matching latest syntax changes)

R=r
CC=golang-dev
https://golang.org/cl/1762042

src/pkg/go/parser/parser.go

@@ -593,9 +593,13 @@ func (p *parser) tryParameterType(ellipsisOk bool) ast.Expr {
 	if ellipsisOk && p.tok == token.ELLIPSIS {
 		pos := p.pos
 		p.next()
-		typ := p.tryType()
+		typ := p.tryType() // don't use parseType so we can provide better error message
+		if typ == nil {
+			p.Error(pos, "'...' parameter is missing type")
+			typ = &ast.BadExpr{pos}
+		}
 		if p.tok != token.RPAREN {
-			p.Error(pos, "can use '...' for last parameter only")
+			p.Error(pos, "can use '...' with last parameter type only")
 		}
 		return &ast.Ellipsis{pos, typ}
 	}

src/pkg/go/parser/parser_test.go

@@ -36,7 +36,7 @@ var validPrograms = []interface{}{
 	`package main; func main() { _ = (<-chan int)(x) }` + "\n",
 	`package main; func main() { _ = (<-chan <-chan int)(x) }` + "\n",
 	`package main; func f(func() func() func())` + "\n",
-	`package main; func f(...)` + "\n",
+	`package main; func f(...T)` + "\n",
 	`package main; func f(float, ...int)` + "\n",
 	`package main; type T []int; var a []bool; func f() { if a[T{42}[0]] {} }` + "\n",
 	`package main; type T []int; func g(int) bool { return true }; func f() { if g(T{42}[0]) {} }` + "\n",