Fix regression with inlined type with methods (#132)

In v2, we report an error if an inlined struct type also has
marshal or unmarshal methods. In such a case, it is ambiguous how
we are supposed to inline the type. The presence of methods
indicates that the type has custom serialization behavior,
but inlining seems to suggest that we should serialize the type
as a Go struct.

Instead of skipping the field, still add the field to the queue
so that we can process the type's fields as if
they were JSON serializable.

Author: dsnet

fields.go

@@ -131,7 +131,6 @@ func makeStructFields(root reflect.Type) (sf structFields, serr *SemanticError)
 				tf := indirectType(f.typ)
 				if implementsAny(tf, allMethodTypes...) && tf != jsontextValueType {
 					serr = orErrorf(serr, t, "inlined Go struct field %s of type %s must not implement marshal or unmarshal methods", sf.Name, tf)
-					continue // invalid inlined field; treat as ignored
 				}
 
 				// Handle an inlined field that serializes to/from

fields_test.go

@@ -261,6 +261,14 @@ func TestMakeStructFields(t *testing.T) {
 		in: struct {
 			A struct{ encoding.TextMarshaler } `json:",inline"`
 		}{},
+		want: structFields{flattened: []structField{{
+			index: []int{0, 0},
+			typ:   reflect.TypeFor[encoding.TextMarshaler](),
+			fieldOptions: fieldOptions{
+				name:       "TextMarshaler",
+				quotedName: `"TextMarshaler"`,
+			},
+		}}},
 		wantErr: errors.New(`inlined Go struct field A of type struct { encoding.TextMarshaler } must not implement marshal or unmarshal methods`),
 	}, {
 		name: jsontest.Name("InlineTextAppender"),
@@ -279,6 +287,14 @@ func TestMakeStructFields(t *testing.T) {
 		in: struct {
 			A struct{ MarshalerTo } `json:",inline"`
 		}{},
+		want: structFields{flattened: []structField{{
+			index: []int{0, 0},
+			typ:   reflect.TypeFor[MarshalerTo](),
+			fieldOptions: fieldOptions{
+				name:       "MarshalerTo",
+				quotedName: `"MarshalerTo"`,
+			},
+		}}},
 		wantErr: errors.New(`inlined Go struct field A of type struct { json.MarshalerTo } must not implement marshal or unmarshal methods`),
 	}, {
 		name: jsontest.Name("UnknownTextUnmarshaler"),
@@ -291,6 +307,14 @@ func TestMakeStructFields(t *testing.T) {
 		in: struct {
 			A *struct{ Unmarshaler } `json:",inline"`
 		}{},
+		want: structFields{flattened: []structField{{
+			index: []int{0, 0},
+			typ:   reflect.TypeFor[Unmarshaler](),
+			fieldOptions: fieldOptions{
+				name:       "Unmarshaler",
+				quotedName: `"Unmarshaler"`,
+			},
+		}}},
 		wantErr: errors.New(`inlined Go struct field A of type struct { json.Unmarshaler } must not implement marshal or unmarshal methods`),
 	}, {
 		name: jsontest.Name("UnknownJSONUnmarshalerFrom"),