func TestPrintDebugPanic(t *testing.T) {
	tt := (*Foo)(nil)
	fmt.Printf("%v", tt)
}

type Foo struct {
	Bar
}

type Bar struct{}

func (Bar) String() string {
	return "bar"
}

	if arg == nil { // <<< return false
		switch verb {
		case 'T', 'v':
			p.fmt.padString(nilAngleString)
		default:
			p.badVerb(verb)
		}
		return
	}

...


		switch verb {
		case 'v', 's', 'x', 'X', 'q':
			// Is it an error or Stringer?
			// The duplication in the bodies is necessary:
			// setting handled and deferring catchPanic
			// must happen before calling the method.
			switch v := p.arg.(type) {
			case error:
				handled = true
				defer p.catchPanic(p.arg, verb, "Error")
				p.fmtString(v.Error(), verb)
				return

			case Stringer:
				handled = true
				defer p.catchPanic(p.arg, verb, "String")
				p.fmtString(v.String(), verb) // <<<<< calls here because the embed Bar implmeent the String()string interface. v is nil
				return
			}
		}