cmd/compile: make for loops with range statements not terminating

Fixes #49003

Change-Id: If09c6f028dce5440b1be238612653ffdd626113a
Reviewed-on: https://go-review.googlesource.com/c/go/+/356189
Trust: Keith Randall <khr@golang.org>
Reviewed-by: roger peppe <rogpeppe@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>

doc/go_spec.html

@@ -1,6 +1,6 @@
 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Version of Sep 16, 2021",
+	"Subtitle": "Version of Oct 15, 2021",
 	"Path": "/ref/spec"
 }-->
 
@@ -4598,7 +4598,8 @@ a <a href="#Blocks">block</a>. The following statements are terminating:
 	A <a href="#For_statements">"for" statement</a> in which:
 	<ul>
 	<li>there are no "break" statements referring to the "for" statement, and</li>
-	<li>the loop condition is absent.</li>
+	<li>the loop condition is absent, and</li>
+	<li>the "for" statement does not use a range clause.</li>
 	</ul>
 </li>
 

src/cmd/compile/internal/types2/return.go

@@ -62,6 +62,11 @@ func (check *Checker) isTerminating(s syntax.Stmt, label string) bool {
 		return true
 
 	case *syntax.ForStmt:
+		if _, ok := s.Init.(*syntax.RangeClause); ok {
+			// Range clauses guarantee that the loop terminates,
+			// so the loop is not a terminating statement. See issue 49003.
+			break
+		}
 		if s.Cond == nil && !hasBreak(s.Body, label, true) {
 			return true
 		}

test/fixedbugs/issue49003.go

@@ -0,0 +1,12 @@
+// errorcheck
+
+// Copyright 2021 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(s string) int {
+	for range s {
+	}
+} // ERROR "missing return"