fix(types): infer response type from last handler in app.on 9-/10-handler overloads

[T4ko0522]

Summary

The 9- and 10-handler overloads of OnHandlerInterface declared a type parameter R on the final handler position but discarded it from the schema computation, hardcoding MergeTypedResponse<HandlerResponse<any>> instead. As a result, the inferred output collapses to {} | Promise<void> and outputFormat to string regardless of what the response handler actually returns, so the RPC client resolves r.json() as Promise<unknown>.

This is a write-omission that was introduced in #1735 (2023-11) and carried through subsequent refactors without being noticed — the surrounding x1–x8 overloads are all using MergeTypedResponse<R> correctly.

Affected overloads

This affects only the x9 / x10 overloads of OnHandlerInterface.
The x1–x8 overloads already use MergeTypedResponse<R> correctly.

Reproduction

import { Hono } from 'hono'
import { hc } from 'hono/client'

const noop = async (_c: any, next: any) => { await next() }

const app = new Hono().on(
  ['GET'], '/api/users/:id',
  noop, noop, noop, noop, noop, noop, noop, noop, noop, // 9 middlewares
  (c) => c.json({ id: '1', name: 'Alice' })             // response handler
)

const client = hc<typeof app>('/')
const data = await client.api.users[':id']
  .$get({ param: { id: '1' } })
  .then((r) => r.json())

// Before this PR: data is `unknown`
// After this PR:  data is `{ id: string; name: string }`

Reducing the chain to 8 handlers makes the inference work correctly even on main, because the x8 overload is not affected by the bug. This bizarre cliff at the x9 boundary is the symptom.

Fix

3-line change in src/types.ts:

- S & ToSchema<M, MergePath<BasePath, P>, I10, MergeTypedResponse<HandlerResponse<any>>>,
+ S & ToSchema<M, MergePath<BasePath, P>, I10, MergeTypedResponse<R>>,

R was already declared in the type parameter list and properly captured by the last H<E11, MergedPath, I10, R> element of the handler tuple — only the schema computation was failing to use it.

Test plan

Added 3 regression tests under describe('OnHandlerInterface', ...) in src/types.test.ts, one per fixed overload. Each test passes 9 or 10 handlers and asserts that the schema's output matches the response handler's return type.

These tests fail on main (the schema's output is {} | Promise<void>) and pass after this fix.

• bun run format:fix && bun run lint:fix
• bun run test (full type check + vitest)
• Added regression tests covering all 3 fixed overloads

Related history

This is not related to #3485 / the .then() arrow-function inference issue, which is a separate TypeScript-level limitation in H = Handler | MiddlewareHandler union inference. This PR fixes an unrelated, mechanical write-omission.

commit	date	note
b80895a — feat(types): Support types until 10 handlers (#1735)	2023-11-28	introduced the hardcoded HandlerResponse<any> in the new x9 / x10 overloads
9732a7b — feat(types): improve ToSchema & WebSocket Helper types (#2588)	2024-05-03	renamed MergeTypedResponseData → MergeTypedResponse, hardcode preserved
37c3809 — fix(types): fix app.on method array type inference (#4578)	2025-12-14	adjacent fix (Ms[number] → M), hardcode preserved

I am using a translation tool. I apologize if any of my expressions cause misunderstanding.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.89%. Comparing base (c37ba26) to head (ba6561a).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4865   +/-   ##
=======================================
  Coverage   92.89%   92.89%           
=======================================
  Files         177      177           
  Lines       11797    11797           
  Branches     3515     3514    -1     
=======================================
  Hits        10959    10959           
  Misses        837      837           
  Partials        1        1           

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[yusukebe]

LGTM!

[yusukebe]

Hi @T4ko0522

The implementation and test are good. Thank you for your contribution!