reflect: add Method.IsExported and StructField.IsExported methods

The IsExported method is a more intuitive helper for checking whether
the method or field is exported than checking whether PkgPath is empty.

In the same CL, modify the standard library to make use of this helper.

Fixes #41563

Change-Id: Iaacfb3b74449501f98e2707aa32095a32bd3c3c1
Reviewed-on: https://go-review.googlesource.com/c/go/+/266197
Trust: Joe Tsai <joetsai@digital-static.net>
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

1 files changed, 2 insertions, 3 deletions

diff --git a/src/encoding/json/encode.go b/src/encoding/json/encode.go
index 483b9d8f2d..751f03d33d 100644
--- a/src/encoding/json/encode.go
+++ b/src/encoding/json/encode.go
@@ -1239,19 +1239,18 @@ func typeFields(t reflect.Type) structFields {
 			// Scan f.typ for fields to include.
 			for i := 0; i < f.typ.NumField(); i++ {
 				sf := f.typ.Field(i)
-				isUnexported := sf.PkgPath != ""
 				if sf.Anonymous {
 					t := sf.Type
 					if t.Kind() == reflect.Ptr {
 						t = t.Elem()
 					}
-					if isUnexported && t.Kind() != reflect.Struct {
+					if !sf.IsExported() && t.Kind() != reflect.Struct {
 						// Ignore embedded fields of unexported non-struct types.
 						continue
 					}
 					// Do not ignore embedded fields of unexported struct types
 					// since they may have exported fields.
-				} else if isUnexported {
+				} else if !sf.IsExported() {
 					// Ignore unexported non-embedded fields.
 					continue
 				}