cmd/compile: fix wrong label loop during rangefunc rewrite

During the rangefunc rewrite, the compiler must correctly identify
the target of branch statements. When a label is defined within a
nested scope - such as inside a function literal or a closure - it
can shadow a label with the same name in the outer scope.

If the rewrite logic does not account for this shadowing, it may
incorrectly associate a branch with a nested label rather than the
intended loop label. Since the typechecker already guarantees that
labels are unique within their respective scopes, any duplicate label
name encountered must belong to a nested scope. These should be
skipped to ensure branch computing correctly targets the current
range-loop scope.

Fixes #78408

Change-Id: I4dce8a4d956f41b3a717a509f8c3f7478720be9f
Reviewed-on: https://go-review.googlesource.com/c/go/+/761420
Reviewed-by: Keith Randall <khr@golang.org>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Jakub Ciolek <jakub@ciolek.dev>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Reviewed-by: Keith Randall <khr@google.com>

src/cmd/compile/internal/rangefunc/rewrite.go

@@ -994,7 +994,14 @@ func (r *rewriter) computeBranchNext() {
 				l := n.Label.Value
 				labels = append(labels, l)
 				f := forStack[len(forStack)-1]
-				r.labelLoop[l] = f
+				if _, existed := r.labelLoop[l]; existed {
+					// The typechecker has already validated that labels are unique within this scope.
+					// If a duplicate label 'X' is encountered, it must reside in a nested scope
+					// (e.g., inside a function literal). Because nested labels do not affect
+					// the current control flow analysis, they can be safely ignored.
+				} else {
+					r.labelLoop[l] = f
+				}
 			}
 		} else {
 			n := stack[len(stack)-1]

test/fixedbugs/issue78408.go

@@ -0,0 +1,18 @@
+// compile
+
+// Copyright 2026 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() {
+A:
+	for range (func(func() bool))(nil) {
+		_ = func() {
+		A:
+			goto A
+		}
+		goto A
+	}
+}