cmd/compile: add cast in range loop final value computation

When replacing a loop where the iteration variable has a named type,
we need to compute the last iteration value as i = T(len(a)-1), not
just i = len(a)-1.

Fixes #73491

Change-Id: Ic1cc3bdf8571a40c10060f929a9db8a888de2b70
Reviewed-on: https://go-review.googlesource.com/c/go/+/667815
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Keith Randall <khr@google.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Reviewed-by: Keith Randall <khr@google.com>

diff --git a/src/cmd/compile/internal/walk/range.go b/src/cmd/compile/internal/walk/range.go
index a1e5442a69eb336fe8ab02907f24968931551518..3d3547b84b500c915ef10291760169162ed0a3ba 100644 (file)
--- a/src/cmd/compile/internal/walk/range.go
+++ b/src/cmd/compile/internal/walk/range.go
@@ -605,7 +605,7 @@ func arrayClear(wbPos src.XPos, a ir.Node, nrange *ir.RangeStmt) ir.Node {
 
        // For array range clear, also set "i = len(a) - 1"
        if nrange != nil {
-               idx := ir.NewAssignStmt(base.Pos, nrange.Key, ir.NewBinaryExpr(base.Pos, ir.OSUB, ir.NewUnaryExpr(base.Pos, ir.OLEN, a), ir.NewInt(base.Pos, 1)))
+               idx := ir.NewAssignStmt(base.Pos, nrange.Key, typecheck.Conv(ir.NewBinaryExpr(base.Pos, ir.OSUB, ir.NewUnaryExpr(base.Pos, ir.OLEN, a), ir.NewInt(base.Pos, 1)), nrange.Key.Type()))
                n.Body.Append(idx)
        }
 

diff --git a/test/fixedbugs/issue73491.go b/test/fixedbugs/issue73491.go
new file mode 100644 (file)
index 0000000..4137088
--- /dev/null
+++ b/test/fixedbugs/issue73491.go
@@ -0,0 +1,25 @@
+// build
+
+// Copyright 2025 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
+
+type T int
+
+const K T = 5
+
+type P struct {
+       a [K]*byte
+}
+
+//go:noinline
+func f(p *P) {
+       for i := range K {
+               p.a[i] = nil
+       }
+}
+func main() {
+       f(nil)
+}