reflect: add example for FieldByIndex

Change-Id: I539453e50ab85ec1b023bc9e329e6451c674e0c9
Reviewed-on: https://go-review.googlesource.com/c/go/+/236937
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Emmanuel Odeke <emmanuel@orijtech.com>
TryBot-Result: Go Bot <gobot@golang.org>

src/reflect/example_test.go

@@ -166,3 +166,31 @@ func ExampleStructOf() {
 	// json:  {"height":0.4,"age":2}
 	// value: &{Height:1.5 Age:10}
 }
+
+func ExampleValue_FieldByIndex() {
+	// This example shows a case in which the name of a promoted field
+	// is hidden by another field: FieldByName will not work, so
+	// FieldByIndex must be used instead.
+	type user struct {
+		firstName string
+		lastName  string
+	}
+
+	type data struct {
+		user
+		firstName string
+		lastName  string
+	}
+
+	u := data{
+		user:      user{"Embedded John", "Embedded Doe"},
+		firstName: "John",
+		lastName:  "Doe",
+	}
+
+	s := reflect.ValueOf(u).FieldByIndex([]int{0, 1})
+	fmt.Println("embedded last name:", s)
+
+	// Output:
+	// embedded last name: Embedded Doe
+}