cmp: guard AllowUnexported against nil arguments

cmp/options.go

@@ -431,7 +431,7 @@ func AllowUnexported(types ...any) Option {
 	m := make(map[reflect.Type]bool)
 	for _, typ := range types {
 		t := reflect.TypeOf(typ)
-		if t.Kind() != reflect.Struct {
+		if t == nil || t.Kind() != reflect.Struct {
 			panic(fmt.Sprintf("invalid struct type: %T", typ))
 		}
 		m[t] = true

cmp/options_test.go

@@ -214,3 +214,19 @@ func TestOptionPanic(t *testing.T) {
 		})
 	}
 }
+
+// TestAllowUnexportedNilPanic asserts that AllowUnexported(nil) panics with
+// the same descriptive message as other invalid inputs rather than crashing
+// with a nil pointer dereference. TestOptionPanic cannot cover this case
+// because it invokes the option functions through reflection, which cannot
+// represent a bare nil argument.
+func TestAllowUnexportedNilPanic(t *testing.T) {
+	var gotPanic any
+	func() {
+		defer func() { gotPanic = recover() }()
+		AllowUnexported(nil)
+	}()
+	if s, ok := gotPanic.(string); !ok || !strings.Contains(s, "invalid struct type") {
+		t.Fatalf("panic = %v (type %T), want string containing %q", gotPanic, gotPanic, "invalid struct type")
+	}
+}