Permit use of IgnoreFields with unexported fields (#203)

Modify IgnoreFields to permit the specification of unexported fields.
To avoid a bug with reflect.Type.FieldByName, we disallow forwarding
on unexported fields. See https://golang.org/issue/4876 for details.

Fixes #196

cmp/cmpopts/struct_filter.go

@@ -160,14 +160,19 @@ func canonicalName(t reflect.Type, sel string) ([]string, error) {
 
 	// Find the canonical name for this current field name.
 	// If the field exists in an embedded struct, then it will be expanded.
+	sf, _ := t.FieldByName(name)
 	if !isExported(name) {
-		// Disallow unexported fields:
-		//	* To discourage people from actually touching unexported fields
-		//	* FieldByName is buggy (https://golang.org/issue/4876)
-		return []string{name}, fmt.Errorf("name must be exported")
+		// Avoid using reflect.Type.FieldByName for unexported fields due to
+		// buggy behavior with regard to embeddeding and unexported fields.
+		// See https://golang.org/issue/4876 for details.
+		sf = reflect.StructField{}
+		for i := 0; i < t.NumField() && sf.Name == ""; i++ {
+			if t.Field(i).Name == name {
+				sf = t.Field(i)
+			}
+		}
 	}
-	sf, ok := t.FieldByName(name)
-	if !ok {
+	if sf.Name == "" {
 		return []string{name}, fmt.Errorf("does not exist")
 	}
 	var ss []string

cmp/cmpopts/util_test.go

@@ -771,6 +771,39 @@ func TestOptions(t *testing.T) {
 		opts:      []cmp.Option{IgnoreFields(Bar3{}, "Bar1", "Bravo", "Delta", "Alpha")},
 		wantEqual: false,
 		reason:    "not equal because highest-level field is not ignored: Foo3",
+	}, {
+		label: "IgnoreFields",
+		x: ParentStruct{
+			privateStruct: &privateStruct{private: 1},
+			PublicStruct:  &PublicStruct{private: 2},
+			private:       3,
+		},
+		y: ParentStruct{
+			privateStruct: &privateStruct{private: 10},
+			PublicStruct:  &PublicStruct{private: 20},
+			private:       30,
+		},
+		opts:      []cmp.Option{cmp.AllowUnexported(ParentStruct{}, PublicStruct{}, privateStruct{})},
+		wantEqual: false,
+		reason:    "not equal because unexported fields mismatch",
+	}, {
+		label: "IgnoreFields",
+		x: ParentStruct{
+			privateStruct: &privateStruct{private: 1},
+			PublicStruct:  &PublicStruct{private: 2},
+			private:       3,
+		},
+		y: ParentStruct{
+			privateStruct: &privateStruct{private: 10},
+			PublicStruct:  &PublicStruct{private: 20},
+			private:       30,
+		},
+		opts: []cmp.Option{
+			cmp.AllowUnexported(ParentStruct{}, PublicStruct{}, privateStruct{}),
+			IgnoreFields(ParentStruct{}, "PublicStruct.private", "privateStruct.private", "private"),
+		},
+		wantEqual: true,
+		reason:    "equal because mismatching unexported fields are ignored",
 	}, {
 		label:     "IgnoreTypes",
 		x:         []interface{}{5, "same"},
@@ -1192,12 +1225,22 @@ func TestPanic(t *testing.T) {
 		args:      args(&Foo1{}, "Alpha"),
 		wantPanic: "must be a struct",
 		reason:    "the type must be a struct (not pointer to a struct)",
+	}, {
+		label:  "IgnoreFields",
+		fnc:    IgnoreFields,
+		args:   args(struct{ privateStruct }{}, "privateStruct"),
+		reason: "privateStruct field permitted since it is the default name of the embedded type",
+	}, {
+		label:  "IgnoreFields",
+		fnc:    IgnoreFields,
+		args:   args(struct{ privateStruct }{}, "Public"),
+		reason: "Public field permitted since it is a forwarded field that is exported",
 	}, {
 		label:     "IgnoreFields",
 		fnc:       IgnoreFields,
-		args:      args(Foo1{}, "unexported"),
-		wantPanic: "name must be exported",
-		reason:    "unexported fields must not be specified",
+		args:      args(struct{ privateStruct }{}, "private"),
+		wantPanic: "does not exist",
+		reason:    "private field not permitted since it is a forwarded field that is unexported",
 	}, {
 		label:  "IgnoreTypes",
 		fnc:    IgnoreTypes,