From d4c729403822eaa0d67ac4fcfc71926a328fcac1 Mon Sep 17 00:00:00 2001
From: "A. Jiang" <de34@live.cn>
Date: Tue, 31 Dec 2024 17:27:52 +0800
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---
 docs/undef.md | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/docs/undef.md b/docs/undef.md
index 4ff6a6b..ac832fb 100644
--- a/docs/undef.md
+++ b/docs/undef.md
@@ -327,12 +327,15 @@ using T1 = decltype(c << 1); // int
 
 ---
 
-对于有符号整数，左移还不得破坏符号位
+C++20 前，有符号整数负值不能左移，而左移正值不能导致结果超出对应的无符号整数类型范围。
 
 ```cpp
-int i = 0;
-i << 1;   // 可以
-i << 31;  // 错！
+int i = 1;
+i << 31; // 可以
+int j = 42;
+j << 31; // C++20 前错！ C++20 起可以
+int k = -1;
+k << 0;  // C++20 前错！ C++20 起可以
 unsigned int u = 0;
 u << 31; // 可以
 ```