reflect: permit unexported fields in StructOf

[dotaheor]

Please answer these questions before submitting your issue. Thanks!

What version of Go are you using (go version)?

go version go1.10.2 linux/amd64

Does this issue reproduce with the latest release?

yes

What did you do?

package main

import "fmt"
import "reflect"

func main() {
	type T = struct {
		x int
	}
	fmt.Println(reflect.TypeOf(T{})) // struct { x int }
	
	var n int
	tn := reflect.TypeOf(n)
	// panic: reflect.StructOf: field "x" is unexported but missing PkgPath
	tt := reflect.StructOf([]reflect.StructField{
		{Name: "x", Type: tn, Anonymous: false},
	})
	fmt.Println(tt)
}

What did you expect to see?

not panic.

What did you see instead?

panic.

If it really should panic, it should be documented.

[gopherbot]

Change https://golang.org/cl/115015 mentions this issue: reflect: document that StructOf panics on unexported fields

[dotaheor]

It looks the modified docs is still not accurate.

StructOf currently does not generate wrapper methods for embedded fields ...

In fact, for gc, if the number of methods is no more than 32 for the new created struct type, the wrapper methods are generated without problems. #25402

package main

import "fmt"
import "reflect"

type T int
func (t T) M() (int, string) {
	return 123, "hello"
}

func main() {
	var t T
	tt := reflect.StructOf([]reflect.StructField{
		{Name: "T", Anonymous: true, Type: reflect.TypeOf(t)},
	})
	
	vtt := reflect.New(tt).Elem()
	fmt.Println(tt.Method(0).Func.Call([]reflect.Value{vtt})) // [<int Value> hello]
	fmt.Println(vtt.Method(0).Call([]reflect.Value{})) // [<int Value> hello]
}

Maybe, the following description is better

StructOf currently does not guarantee to generate wrapper methods for embedded fields ...

[purpleidea]

Can we get this fixed so it is allowed, or learn why it is not? (I think it should be.)

[ianlancetaylor]

Reopening to permit this case.