fix: panic when decoding time.Time with context

close #327

decode_test.go

@@ -3828,3 +3828,17 @@ func TestIssue303(t *testing.T) {
 		t.Fatalf("failed to decode. count = %d type = %s value = %v", v.Count, v.Type, v.Value)
 	}
 }
+
+func TestIssue327(t *testing.T) {
+	var v struct {
+		Date time.Time `json:"date"`
+	}
+	dec := json.NewDecoder(strings.NewReader(`{"date": "2021-11-23T13:47:30+01:00"})`))
+	if err := dec.DecodeContext(context.Background(), &v); err != nil {
+		t.Fatal(err)
+	}
+	expected := "2021-11-23T13:47:30+01:00"
+	if got := v.Date.Format(time.RFC3339); got != expected {
+		t.Fatalf("failed to decode. expected %q but got %q", expected, got)
+	}
+}

internal/decoder/unmarshal_json.go

@@ -1,6 +1,7 @@
 package decoder
 
 import (
+	"context"
 	"encoding/json"
 	"unsafe"
 
@@ -46,13 +47,20 @@ func (d *unmarshalJSONDecoder) DecodeStream(s *Stream, depth int64, p unsafe.Poi
 		typ: d.typ,
 		ptr: p,
 	}))
+	switch v := v.(type) {
+	case unmarshalerContext:
+		var ctx context.Context
 		if (s.Option.Flags & ContextOption) != 0 {
-		if err := v.(unmarshalerContext).UnmarshalJSON(s.Option.Context, dst); err != nil {
+			ctx = s.Option.Context
+		} else {
+			ctx = context.Background()
+		}
+		if err := v.UnmarshalJSON(ctx, dst); err != nil {
 			d.annotateError(s.cursor, err)
 			return err
 		}
-	} else {
-		if err := v.(json.Unmarshaler).UnmarshalJSON(dst); err != nil {
+	case json.Unmarshaler:
+		if err := v.UnmarshalJSON(dst); err != nil {
 			d.annotateError(s.cursor, err)
 			return err
 		}