diff --git a/lib/core/Handles/HandlesAnnAssign.py b/lib/core/Handles/HandlesAnnAssign.py
index c3cb6f2..801d911 100644
--- a/lib/core/Handles/HandlesAnnAssign.py
+++ b/lib/core/Handles/HandlesAnnAssign.py
@@ -281,8 +281,19 @@ class AnnAssignHandle(BaseHandle):
                         # 不是 trunc 取最低位（运行时值 2 会得 false，与 `if 2:` 相反）
                         if TargetType.width == 1:
                             Value = Gen._CoerceIntToI1(Value, name="bool_norm")
+                        # 隐式窄化**禁止**（与 MemoryOps._coerce_value 同口径，规则 2026-10-10）：
+                        # 位宽高→低会丢高位，必须显式 cast 或加宽目标类型。
+                        # 唯一豁免：源是**编译期常量且值落进目标位宽**（无信息损失）。
+                        elif (isinstance(Value, ir.Constant) and isinstance(Value.constant, int)
+                              and -(1 << (TargetType.width - 1)) <= Value.constant <= (1 << TargetType.width) - 1):
+                            Value = Gen.builder.trunc(Value, TargetType, name="trunc_const")
                         else:
-                            Value = Gen.builder.trunc(Value, TargetType, name="trunc")
+                            from lib.core.LLVMCG.FuncGen import FailFastError
+                            raise FailFastError(
+                                f"隐式窄化禁止：{Value.type.width} 位整数不能隐式赋给 {TargetType.width} 位类型"
+                                f"（会丢高位）；请显式转换（如 int32(x) / t.CInt32T(x) ）或把目标类型加宽"
+                                f"{Gen._DescribeValueNode(Node)}"
+                            )
                     else:
                         Value = Gen.builder.zext(Value, TargetType, name="zext")
                 elif isinstance(TargetType, ir.PointerType) and isinstance(Value.type, ir.PointerType):