Print type name in unexported panic (#171)

In the panic message when accessing an unexported field,
print the full name of the type for user convenience.

cmp/compare_test.go

@@ -15,6 +15,7 @@ import (
 	"reflect"
 	"regexp"
 	"sort"
+	"strconv"
 	"strings"
 	"sync"
 	"testing"
@@ -119,6 +120,10 @@ func comparerTests() []test {
 		Xattrs     map[string]string
 	}
 
+	type namedWithUnexported struct {
+		unexported string
+	}
+
 	makeTarHeaders := func(tf byte) (hs []tarHeader) {
 		for i := 0; i < 5; i++ {
 			hs = append(hs, tarHeader{
@@ -673,6 +678,18 @@ func comparerTests() []test {
   }
 `,
 		reason: "all zero map entries are ignored (even if missing)",
+	}, {
+		label:     label,
+		x:         namedWithUnexported{},
+		y:         namedWithUnexported{},
+		wantPanic: strconv.Quote(reflect.TypeOf(namedWithUnexported{}).PkgPath()) + ".namedWithUnexported",
+		reason:    "panic on named struct type with unexported field",
+	}, {
+		label:     label,
+		x:         struct{ a int }{},
+		y:         struct{ a int }{},
+		wantPanic: strconv.Quote(reflect.TypeOf(namedWithUnexported{}).PkgPath()) + ".(struct { a int })",
+		reason:    "panic on unnamed struct type with unexported field",
 	}}
 }
 

cmp/options.go

@@ -226,7 +226,19 @@ func (validator) apply(s *state, vx, vy reflect.Value) {
 	// Unable to Interface implies unexported field without visibility access.
 	if !vx.CanInterface() || !vy.CanInterface() {
 		const help = "consider using a custom Comparer; if you control the implementation of type, you can also consider AllowUnexported or cmpopts.IgnoreUnexported"
-		panic(fmt.Sprintf("cannot handle unexported field: %#v\n%s", s.curPath, help))
+		var name string
+		if t := s.curPath.Index(-2).Type(); t.Name() != "" {
+			// Named type with unexported fields.
+			name = fmt.Sprintf("%q.%v", t.PkgPath(), t.Name()) // e.g., "path/to/package".MyType
+		} else {
+			// Unnamed type with unexported fields. Derive PkgPath from field.
+			var pkgPath string
+			for i := 0; i < t.NumField() && pkgPath == ""; i++ {
+				pkgPath = t.Field(i).PkgPath
+			}
+			name = fmt.Sprintf("%q.(%v)", pkgPath, t.String()) // e.g., "path/to/package".(struct { a int })
+		}
+		panic(fmt.Sprintf("cannot handle unexported field at %#v:\n\t%v\n%s", s.curPath, name, help))
 	}
 
 	panic("not reachable")