Case Study: The C++20 Migration Compiled Clean. A 40-Line Property Test Found the Ordering Break.

说明与用途

将库迁移到 C++20 改变了记录的排序顺序，只有通过 40 行属性测试才发现。测试表明，自动 diff 将手写的 operator< 替换为默认的 <=>，改变了成员顺序，从而改变了排序。

问题

Migrating a 2,400‑line library to C++20 changed the sort order of report records, breaking callers that relied on a specific priority‑timestamp‑name ordering.

结果

The property test revealed that the defaulted comparison changed the ordering, exposing a regression that unit tests missed.

工作流

• Snapshot existing test outputs’ SHA‑256
• Prompt model to apply only mechanical changes
• Apply diff and build with -std=c++20 -Wall -Wextra -Werror
• Run existing 14 unit tests
• Add ordering property test
• Run whole job under ASan and UBSan

证据

The model’s reasoning was defensible. A defaulted <= > generates all six comparison operators, and operator< is synthesized from it.

来源

The risky 13% was a single change to Record.