fix: confusing nil in direct interface with typed nil

fix #376

encode_test.go

@@ -2406,3 +2406,21 @@ func TestIssue339(t *testing.T) {
 		t.Errorf("unexpected result: %v != %v", got, expected)
 	}
 }
+
+func TestIssue376(t *testing.T) {
+	type Container struct {
+		V interface{} `json:"value"`
+	}
+	type MapOnly struct {
+		Map map[string]int64 `json:"map"`
+	}
+	b, err := json.Marshal(Container{MapOnly{}})
+	if err != nil {
+		t.Fatal(err)
+	}
+	got := string(b)
+	expected := `{"value":{"map":null}}`
+	if got != expected {
+		t.Errorf("unexpected result: %v != %v", got, expected)
+	}
+}

internal/cmd/generator/vm.go.tmpl

@@ -3,6 +3,7 @@ package vm
 
 import (
 	"math"
+	"reflect"
 	"sort"
 	"unsafe"
 
@@ -193,7 +194,8 @@ func Run(ctx *encoder.RuntimeContext, b []byte, codeSet *encoder.OpcodeSet) ([]b
 				ifacePtr = iface.ptr
 				typ = iface.typ
 			}
-			if ifacePtr == nil {
+			isDirectedNil := typ != nil && typ.Kind() == reflect.Struct && !runtime.IfaceIndir(typ)
+			if ifacePtr == nil && !isDirectedNil {
 				b = appendNullComma(ctx, b)
 				code = code.Next
 				break

internal/encoder/vm/vm.go

@@ -3,6 +3,7 @@ package vm
 
 import (
 	"math"
+	"reflect"
 	"sort"
 	"unsafe"
 
@@ -193,7 +194,8 @@ func Run(ctx *encoder.RuntimeContext, b []byte, codeSet *encoder.OpcodeSet) ([]b
 				ifacePtr = iface.ptr
 				typ = iface.typ
 			}
-			if ifacePtr == nil {
+			isDirectedNil := typ != nil && typ.Kind() == reflect.Struct && !runtime.IfaceIndir(typ)
+			if ifacePtr == nil && !isDirectedNil {
 				b = appendNullComma(ctx, b)
 				code = code.Next
 				break

internal/encoder/vm_color/vm.go

@@ -3,6 +3,7 @@ package vm_color
 
 import (
 	"math"
+	"reflect"
 	"sort"
 	"unsafe"
 
@@ -193,7 +194,8 @@ func Run(ctx *encoder.RuntimeContext, b []byte, codeSet *encoder.OpcodeSet) ([]b
 				ifacePtr = iface.ptr
 				typ = iface.typ
 			}
-			if ifacePtr == nil {
+			isDirectedNil := typ != nil && typ.Kind() == reflect.Struct && !runtime.IfaceIndir(typ)
+			if ifacePtr == nil && !isDirectedNil {
 				b = appendNullComma(ctx, b)
 				code = code.Next
 				break

internal/encoder/vm_color_indent/vm.go

@@ -3,6 +3,7 @@ package vm_color_indent
 
 import (
 	"math"
+	"reflect"
 	"sort"
 	"unsafe"
 
@@ -193,7 +194,8 @@ func Run(ctx *encoder.RuntimeContext, b []byte, codeSet *encoder.OpcodeSet) ([]b
 				ifacePtr = iface.ptr
 				typ = iface.typ
 			}
-			if ifacePtr == nil {
+			isDirectedNil := typ != nil && typ.Kind() == reflect.Struct && !runtime.IfaceIndir(typ)
+			if ifacePtr == nil && !isDirectedNil {
 				b = appendNullComma(ctx, b)
 				code = code.Next
 				break

internal/encoder/vm_indent/vm.go

@@ -3,6 +3,7 @@ package vm_indent
 
 import (
 	"math"
+	"reflect"
 	"sort"
 	"unsafe"
 
@@ -193,7 +194,8 @@ func Run(ctx *encoder.RuntimeContext, b []byte, codeSet *encoder.OpcodeSet) ([]b
 				ifacePtr = iface.ptr
 				typ = iface.typ
 			}
-			if ifacePtr == nil {
+			isDirectedNil := typ != nil && typ.Kind() == reflect.Struct && !runtime.IfaceIndir(typ)
+			if ifacePtr == nil && !isDirectedNil {
 				b = appendNullComma(ctx, b)
 				code = code.Next
 				break