go/types, types2: ensure deterministic output when reporting an init cycle

Fixes #71254

Author: wingrez

src/cmd/compile/internal/types2/initorder.go

@@ -139,10 +139,24 @@ func findPath(objMap map[Object]*declInfo, from, to Object, seen map[Object]bool
 	}
 	seen[from] = true
 
-	for d := range objMap[from].deps {
-		if d == to {
-			return []Object{d}
-		}
+	// premature optimization - leave away
+	// findPath only gets called when there's an error.
+	if objMap[from].deps[to] {
+		return []Object{to}
+	}
+
+	// Sort deps here in order to return a deterministic path,
+	// thereby guaranteeing consistent output in reportCycle.
+	// Performance is never critical when we have an error, leave away.
+	deps := make([]Object, 0, len(objMap[from].deps))
+	for dep := range objMap[from].deps {
+		deps = append(deps, dep)
+	}
+	slices.SortFunc(deps, func(x, y Object) int {
+		return cmp.Compare(x.order(), y.order())
+	})
+
+	for _, d := range deps {
 		if P := findPath(objMap, d, to, seen); P != nil {
 			return append(P, d)
 		}

src/go/types/initorder.go

@@ -0,0 +1,14 @@
+// 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 p
+
+const (
+	B /* ERROR "initialization cycle: B refers to itself" */ = A + B
+	A /* ERROR "initialization cycle: A refers to itself" */ = A + B
+
+	C /* ERRORx "initialization cycle for C\\s+.*C refers to D\\s+.*D refers to C" */ = E + D
+	D /* ERRORx "initialization cycle for D\\s+.*D refers to C\\s+.*C refers to D" */ = E + C
+	E = D + C
+)