fix: confusing nil in direct interface with typed nil

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"
 
@@ -194,9 +195,12 @@ func Run(ctx *encoder.RuntimeContext, b []byte, codeSet *encoder.OpcodeSet) ([]b
 				typ = iface.typ
 			}
 			if ifacePtr == nil {
-				b = appendNullComma(ctx, b)
-				code = code.Next
-				break
+				isDirectedNil := typ != nil && typ.Kind() == reflect.Struct && !runtime.IfaceIndir(typ)
+				if !isDirectedNil {
+					b = appendNullComma(ctx, b)
+					code = code.Next
+					break
+				}
 			}
 			ctx.KeepRefs = append(ctx.KeepRefs, up)
 			ifaceCodeSet, err := encoder.CompileToGetCodeSet(ctx, uintptr(unsafe.Pointer(typ)))