diff --git a/App/lib/core/Gen/ExprGenAttr.vp b/App/lib/core/Gen/ExprGenAttr.vp
index 993cf4f..9ac1431 100644
--- a/App/lib/core/Gen/ExprGenAttr.vp
+++ b/App/lib/core/Gen/ExprGenAttr.vp
@@ -789,6 +789,18 @@ class ExprGenAttr(ExprGenBase.ExprGenBase):
                 # 查找字段索引和类型
                 field_info: GenTypes.FieldEntry | t.CPtr = self.Gen.StructGen.lookup_field(
                     struct_ty, at.attr)
+                # [GAP-DIAG] 定位 th.arg=arg 字段索引错（func/arg 都解析成 3）根因：
+                # 打印字段名/结构体类型名/查到的索引与实际字段名
+                if field_info is not None \
+                        and (string.strcmp(at.attr, "arg") == 0 or string.strcmp(at.attr, "func") == 0):
+                    dbg_gap: str = pool.alloc(200)
+                    if dbg_gap is not None:
+                        ty_nm_gap: str = self.Gen.StructGen._extract_struct_name(struct_ty)
+                        if ty_nm_gap is None:
+                            ty_nm_gap = "?"
+                        viperlib.snprintf(dbg_gap, 200, "[GAP] attr=%s ty=%s idx=%d fe_name=%s\n",
+                                          at.attr, ty_nm_gap, field_info.Index, field_info.Name)
+                        stdio.printf("%s", dbg_gap)
                 # REnum 变体字段查找: lookup_field 失败时，尝试 REnum 别名
                 # REnum 结构体字段名是 _p1/_p2/...，但源码用变体字段名访问（如 ty.Pointee）
                 if field_info is None: