Unreachable Code

Unreachable code cannot be executed by any input. In its obvious form it is code after a return. In its interesting form it is a guard, a fallback, or an error branch protecting against a state the surrounding design makes impossible.

Unreachable branches are worth eliminating rather than tolerating for two reasons. They are noise — a reader cannot distinguish a real constraint from a defensive reflex, which is Chesterton's Fence in miniature. And under a strict Coverage Gate they are unfixable by testing, since no test can reach them.

Some recurring generators, and their structural fixes:

• A strict compiler setting making indexed access possibly-undefined forces a guard even when the key came from the same map's own keys. Iterating over entries rather than keys removes it, because the tuple type is not optional.
• A defensive null check on a reference whose invariant guarantees it exists. A non-null assertion compiles to nothing and removes the branch.
• Independent optionals that always travel together — a URL and its alternative text — create a "one present, one absent" case that no caller can produce. Modelling them as a single both-or-neither unit deletes it.
• A default value for a parameter every caller supplies.

Each fix removes the branch rather than testing it, which is the distinction that matters.