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>

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)
 	}
 

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)
+}