# Matcher.Error

Extracts a Matcher's declared error type.

## Signatures

```ts
type Error<T> = [
    T
] extends [
    Matcher<infer _A, infer E, infer _R>
] ? E : never;
```

## Why

Router builders can transform errors without widening away individual variants.

## Ownership and lifetime

Computed only by TypeScript; it does not retain a Matcher or any failure value.