util/cmpx: add package with cmp-like things from future Go releases

Updates #8296

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

util/cmpx/cmpx.go

@@ -0,0 +1,22 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Package cmpx has code that will likely land in a future version of Go, but
+// we want sooner.
+package cmpx
+
+// Or returns the first non-zero element of list, or else returns the zero T.
+//
+// This is the proposal from
+// https://github.com/golang/go/issues/60204#issuecomment-1581245334.
+func Or[T comparable](list ...T) T {
+	// TODO(bradfitz): remove the comparable constraint so we can use this
+	// with funcs too and use reflect to see whether they're non-zero? 🤷‍♂️
+	var zero T
+	for _, v := range list {
+		if v != zero {
+			return v
+		}
+	}
+	return zero
+}

util/cmpx/cmpx_test.go

@@ -0,0 +1,24 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package cmpx
+
+import "testing"
+
+func TestOr(t *testing.T) {
+	if g, w := Or[string](), ""; g != w {
+		t.Errorf("got %v; want %v", g, w)
+	}
+	if g, w := Or[int](), 0; g != w {
+		t.Errorf("got %v; want %v", g, w)
+	}
+	if g, w := Or("", "foo", "bar"), "foo"; g != w {
+		t.Errorf("got %v; want %v", g, w)
+	}
+	if g, w := Or("foo", "bar"), "foo"; g != w {
+		t.Errorf("got %v; want %v", g, w)
+	}
+	if g, w := Or("", "", "bar"), "bar"; g != w {
+		t.Errorf("got %v; want %v", g, w)
+	}
+}