From be5a66e0a0e9061d33ec0dece356d7ac552924a9 Mon Sep 17 00:00:00 2001
From: maru44 <takumaru22244@gmail.com>
Date: Mon, 4 Apr 2022 22:34:13 +0900
Subject: [PATCH] encode

---
 src/encoding/json/encode.go      | 16 +++++++-
 src/encoding/json/encode_test.go | 69 ++++++++++++++++++++++++++++++++
 2 files changed, 84 insertions(+), 1 deletion(-)

diff --git a/src/encoding/json/encode.go b/src/encoding/json/encode.go
index fc865386ed37c2..3f8c3607bc916d 100644
--- a/src/encoding/json/encode.go
+++ b/src/encoding/json/encode.go
@@ -1239,6 +1239,8 @@ func typeFields(t reflect.Type) structFields {
 
 			// Scan f.typ for fields to include.
 			for i := 0; i < f.typ.NumField(); i++ {
+				var spread bool
+
 				sf := f.typ.Field(i)
 				if sf.Anonymous {
 					t := sf.Type
@@ -1259,6 +1261,18 @@ func typeFields(t reflect.Type) structFields {
 				if tag == "-" {
 					continue
 				}
+				if tag == "..." {
+					t := sf.Type
+					spread = true
+					if t.Kind() == reflect.Pointer {
+						t = t.Elem()
+					}
+					if !sf.IsExported() && t.Kind() != reflect.Struct {
+						// Ignore embedded fields of unexported non-struct types.
+						continue
+					}
+				}
+
 				name, opts := parseTag(tag)
 				if !isValidTag(name) {
 					name = ""
@@ -1287,7 +1301,7 @@ func typeFields(t reflect.Type) structFields {
 				}
 
 				// Record found field and index sequence.
-				if name != "" || !sf.Anonymous || ft.Kind() != reflect.Struct {
+				if !spread && (name != "" || !sf.Anonymous || ft.Kind() != reflect.Struct) {
 					tagged := name != ""
 					if name == "" {
 						name = sf.Name
diff --git a/src/encoding/json/encode_test.go b/src/encoding/json/encode_test.go
index 0b021f0074991d..ac398478bd5716 100644
--- a/src/encoding/json/encode_test.go
+++ b/src/encoding/json/encode_test.go
@@ -1201,3 +1201,72 @@ func TestMarshalerError(t *testing.T) {
 		}
 	}
 }
+
+func TestMarshalSpread(t *testing.T) {
+	type base[T any] struct {
+		Name      string
+		Free      T `json:"..."`
+		CreatedAt time.Time
+	}
+
+	type additional struct {
+		Age     int
+		IsValid bool
+	}
+
+	now := time.Now()
+
+	tests := []struct{
+		name string
+		in any
+		want string
+	}{
+		{
+			name: "ok",
+			in: base[additional]{
+				Name: "Foo",
+				Free: additional{
+					Age: 20,
+				}
+				CreatedAt: now,
+			},
+			want: fmt.Sprintf(`{"Name":"Foo","Age":20,"IsValid":false,"CreatedAt":"%s"}`, now.String()),
+		},
+		{
+			name: "ok ptr",
+			in: base[*additional]{
+				Name: "Foo",
+				Free: &additional{
+					Age: 40,
+					IsValid: true,
+				},
+				CreatedAt: now,
+			}
+			want: fmt.Sprintf(`{"Name":"Foo","Age":40,"IsValid":true,"CreatedAt":"%s"}`, now.String()),
+		},
+		{
+			name: "ok ignored",
+			in: base[string]{
+				Name: "Foo",
+				Free: "ignored...",
+				CreatedAt: now,
+			}
+			want: fmt.Sprintf(`{"Name":"Foo","CreatedAt":"%s"}`, now.String()),
+		},
+	}
+
+	for _, tt := range tests{
+		tt := tt
+		b, err := Marshal(tt.in)
+		if ok := (err == nil); ok != tt.ok {
+			if err != nil {
+				t.Errorf("test %d, unexpected failure: %v", i, err)
+			} else {
+				t.Errorf("test %d, unexpected success", i)
+			}
+		}
+		if got := string(b); got != tt.want {
+			t.Errorf("test %d, Marshal(%#v) = %q, want %q", i, tt.in, got, tt.want)
+		}
+	}
+}