使用％v格式调用String（）以获取嵌套结构

In the following code, I would expect fmt.Printf("%v ", a) to invoke String() of its member of type myTypeB, but this is not happening ? Why ?

package main

import "fmt"
type myTypeA struct {
     b myTypeB
}
type myTypeB struct {
    c string
    d int

}

func (b myTypeB) String() string {
    return "myTypeB custom"
}

func main() {

   a:= myTypeA{myTypeB{"hello", 1}};
   b:= myTypeB{"hello", 1}
   fmt.Printf("%v
", a)
   fmt.Printf("%v
", b)
}

Playground link

fmt doesn't look for fmt.Stringer recursively.

If the argument is a fmt.Stringer, it will invoke the String() method, and print the results. If it doesn't have a String() method, fmt will iterate over the fields using reflection to obtain the value.