Guarding a regression

A guard you have not watched fail is not a guard — it is a line of code that runs. Two moves make it real, both required: can you drive the system into the broken state, and can you see that state from where you assert?

The default guard is written after the fix, at the layer just edited, never observed red. All three defaults go green.

When to use

• You fixed a bug and are about to write the test that pins it.
• You are adding a lint rule, schema check, validating constructor or CI step and saying it prevents something.
• You are reviewing a regression test and cannot tell what would make it fail.
• A bug you fixed came back, and a test covering it was green throughout.

When NOT to use