cmd/compile: use the zero value for results of impossible indexing

type A [0]int
var a A
x := a[i]

Use the zero value for x instead of the "impossible" value. That lets
us at least compile code like this with -B, even though it can't
possibly run correctly.

Fixes #48092

Change-Id: Idad5cfab49e05f375c069b05addceed68a15299f
Reviewed-on: https://go-review.googlesource.com/c/go/+/346589
Trust: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Go Bot <gobot@golang.org>

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

@@ -3014,7 +3014,8 @@ func (s *state) expr(n ir.Node) *ssa.Value {
 					z := s.constInt(types.Types[types.TINT], 0)
 					s.boundsCheck(z, z, ssa.BoundsIndex, false)
 					// The return value won't be live, return junk.
-					return s.newValue0(ssa.OpUnknown, n.Type())
+					// But not quite junk, in case bounds checks are turned off. See issue 48092.
+					return s.zeroVal(n.Type())
 				}
 				len := s.constInt(types.Types[types.TINT], bound)
 				s.boundsCheck(i, len, ssa.BoundsIndex, n.Bounded()) // checks i == 0

test/fixedbugs/issue48092.go

@@ -0,0 +1,17 @@
+// compile -B
+
+// 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.
+
+// Make sure that we can at least compile this code
+// successfully with -B. We can't ever produce the right
+// answer at runtime with -B, as the access must panic.
+
+package p
+
+type A [0]byte
+
+func (a *A) Get(i int) byte {
+	return a[i]
+}