cmd/compile: add missing bound checks when handle zero-sized values

Fixes #79197

Change-Id: Ifded6ec4c014d2e87dab7b66c8063f8ffa66be5d
Reviewed-on: https://go-review.googlesource.com/c/go/+/774120
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@golang.org>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: David Chase <drchase@google.com>

src/cmd/compile/internal/ssagen/ssa.go

@@ -4549,6 +4549,8 @@ func (s *state) assignWhichMayOverlap(left ir.Node, right *ssa.Value, deref bool
 				return
 			}
 			if t.Size() == 0 {
+				len := s.constInt(types.Types[types.TINT], n)
+				s.boundsCheck(i, len, ssa.BoundsIndex, false)
 				return
 			}
 

test/fixedbugs/issue79197.go

@@ -0,0 +1,41 @@
+// run
+
+// 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 main
+
+//go:noinline
+func a1(i int) {
+	var a [1]struct{}
+	a[i] = struct{}{}
+}
+
+//go:noinline
+func a2(i int) {
+	var a [1][0]int
+	a[i] = [0]int{}
+}
+
+//go:noinline
+func a3(i int) {
+	var a [1]struct{ x [0]int }
+	a[i] = struct{ x [0]int }{}
+}
+
+func wantPanic(name string, f func()) {
+	defer func() {
+		if r := recover(); r != nil {
+			return
+		}
+		panic(name + ": no panic (bug)")
+	}()
+	f()
+}
+
+func main() {
+	wantPanic("a1", func() { a1(5) })
+	wantPanic("a2", func() { a2(5) })
+	wantPanic("a3", func() { a3(5) })
+}