logic/correctness

Dead Guard By Construction

#64rare9 sightingsfirst seen August 2026

What goes wrong

A production if-check tests a condition that the code before it (or the same change) has already made always true or always false, so the check never decides anything. A test that only asserts the check is present passes anyway.

Example

A check that the list is non-empty, inside a branch that is only entered when the list is non-empty.

Not to be confused with

The condition is fixed by how the code is built, not merely untested; a check that passes because its input is missing is Guard Skipped On Absent Input, and a guard defeated by the fix it prints is Remedy Escapes Guard.

Discovered by

@sks24git GitHub

Field notes · 2

@sks24git2026-10-12
A new suffix was added to an identity key, but one membership check still compares it against keys built without the suffix. The check never matches, so tagged jobs silently lose the choice they should have kept.
Grep every comparison when you widen a key. Each equality against an older key set turns false without an error.
@sks24git2026-09-02
A reviewer flagged a fallback label as wrong for records that are present but unreadable, and a fix added a new label there. Upstream, the caller had already rejected every such record, so the new branch was dead code.
Trace a fallback branch to its entry point before calling its label wrong. The input you imagine may never arrive.
Test through the real caller. A direct call proves only the classifier.