fix: restore Result-inferring andThrough overload on Err

[chatman-media]

Problem

Closes #663.

andThrough reports a spurious type error (and drops error types) when it is chained after a method that widens the receiver to the Result<T, E> union — e.g. a preceding .andThen():

class ErrorA extends Error { readonly _tag = 'ErrorA' as const }
class ErrorB extends Error { readonly _tag = 'ErrorB' as const }

const assertPublishable = (s: { title: string; description: string }) => {
  if (!s.title) return err(new ErrorA())
  if (!s.description) return err(new ErrorB())
  return ok(undefined)
}

ok({ title: 'title', description: 'description' })
  .andThen((s) => ok(s))      // widens receiver to Result<T, never>
  .andThrough(assertPublishable) // ❌ TS2345 before this fix

Curiously, removing the .andThen((s) => ok(s)) line makes the error disappear and the type infers correctly as Result<{ title, description }, ErrorA | ErrorB>.

Root cause

andThrough is declared with two overloads on the IResult interface and on the Ok class:

andThrough<R extends Result<unknown, unknown>>(f: (t: T) => R): Result<T, InferErrTypes<R> | E>
andThrough<F>(f: (t: T) => Result<unknown, F>): Result<T, E | F>

but the Err class only declared the second one:

andThrough<F>(_f: (t: T) => Result<unknown, F>): Result<T, E | F> { ... }

When andThrough is called on a value typed as the union Ok<T, E> | Err<T, E> (which is what Result<T, E> is, and what .andThen() produces), TypeScript resolves the call against the signatures common to both union members. Because Err was missing the first overload, resolution fell through to andThrough<F>(...), which infers F from only the first member of a union-returning callback (ErrorA) and discards the rest — producing (t) => Result<unknown, ErrorA> and the resulting mismatch.

Calling andThrough directly on an Ok (which has both overloads) already worked, which is why the error only appears once the chain has widened to Result.

Fix

Add the missing andThrough<R extends Result<unknown, unknown>> overload to the Err class so it matches Ok and the IResult interface. The runtime implementation is unchanged (Err.andThrough still just returns err(this.error)).

Tests

Added a type-level regression test in tests/typecheck-tests.ts under the andThrough block. It reproduces the issue's exact shape (a .andThen(...) followed by .andThrough(...) with a union-returning callback) and asserts the inferred type is Result<{ title, description }, ErrorA | ErrorB>. The test fails (TS2345) on master and passes with this change.

• npm test — 117 runtime tests pass, type tests pass
• npm run lint / npm run typecheck — clean

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