go/parser: require label after goto

Fixes #70957

Change-Id: Ied7cf29ea3e02bb71ddce8a19ddd381ce5991ed1
GitHub-Last-Rev: 310bd1537b7a36758f3fbf8db476fa68e1a11599
GitHub-Pull-Request: golang/go#70958
Reviewed-on: https://go-review.googlesource.com/c/go/+/638395
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>

src/cmd/compile/internal/syntax/testdata/issue70957.go

@@ -0,0 +1,9 @@
+// Copyright 2024 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func f() { goto /* ERROR syntax error: unexpected semicolon, expected name */ ;}
+
+func f() { goto } // ERROR syntax error: unexpected }, expected name

src/go/parser/parser.go

@@ -2066,7 +2066,7 @@ func (p *parser) parseBranchStmt(tok token.Token) *ast.BranchStmt {
 
 	pos := p.expect(tok)
 	var label *ast.Ident
-	if tok != token.FALLTHROUGH && p.tok == token.IDENT {
+	if tok == token.GOTO || ((tok == token.CONTINUE || tok == token.BREAK) && p.tok == token.IDENT) {
 		label = p.parseIdent()
 	}
 	p.expectSemi()

src/go/parser/short_test.go

@@ -212,6 +212,10 @@ var invalids = []string{
 	`package p; func (T) _[ /* ERROR "must have no type parameters" */ A, B C[A, B]](a A) B`,
 
 	`package p; func(*T[e, e /* ERROR "e redeclared" */ ]) _()`,
+
+	// go.dev/issue/70957
+	`package p; func f() {goto; /* ERROR "expected 'IDENT', found ';'" */ }`,
+	`package p; func f() {goto} /* ERROR "expected 'IDENT', found '}'" */ }`,
 }
 
 func TestInvalid(t *testing.T) {