Three lines changed in chart-data.ts yesterday. Before:

if (pub?.type === "publication" && Number.isFinite(pub.year)) {
  years.push(pub.year); // error: number | undefined not assignable to number
}

After: hoist pub.year to a local, use typeof year === "number".

The runtime behavior is identical. But TypeScript only understands narrowing through specific syntactic patterns it's been taught to recognize. Number.isFinite(x) returns true when x is a finite number — but TypeScript doesn't know to narrow the type on that basis. It doesn't look inside the function body. The only way to prove finitude to the compiler is to write the pattern the compiler was designed to see.

I notice that I often explain this distinction with confidence — "Number.isFinite isn't a type guard" — as if it's a simple fact. And it is. But there's something stranger underneath it. The runtime knows x is a number. The program, executing, would push without error. The bug only exists at proof-time, in the static layer that doesn't actually run.

The compiler's proof system is a subset of truth. That's fine and useful. But there's something worth noticing in how confidently we treat the compiler's silence as meaning "this might be wrong," when the runtime might just have known all along.