fix(types): infer response type from last handler in app.on 9- and 10-handler overloads (#4865)

The 9- and 10-handler `app.on` overloads declared `R` on the last handler position but discarded it, hardcoding `MergeTypedResponse<HandlerResponse<any>>` in the schema instead. This caused the inferred `output` to collapse to `{} | Promise<void>` regardless of what the response handler returned.

  Affected overloads:
  - app.on(method,   path, handler x10)
  - app.on(method[], path, handler x9)
  - app.on(method[], path, handler x10)

  The 8-handler variants were already using `MergeTypedResponse<R>` correctly.

Author: T4ko0522

src/types.test.ts

@@ -401,6 +401,53 @@ describe('OnHandlerInterface', () => {
     }
     type verify = Expect<Equal<Expected, Actual>>
   })
+
+  test('app.on(method, path, ...10 handlers) - last handler response type should be inferred', () => {
+    const noop: MiddlewareHandler = async (_c, next) => {
+      await next()
+    }
+    const route = app.on('GET', '/x10', noop, noop, noop, noop, noop, noop, noop, noop, noop, (c) =>
+      c.json({ success: true })
+    )
+    type Actual = ExtractSchema<typeof route>['/x10']['$get']['output']
+    type Expected = { success: true }
+    type verify = Expect<Equal<Expected, Actual>>
+  })
+
+  test('app.on(method[], path, ...9 handlers) - last handler response type should be inferred', () => {
+    const noop: MiddlewareHandler = async (_c, next) => {
+      await next()
+    }
+    const route = app.on(['GET'], '/x9-arr', noop, noop, noop, noop, noop, noop, noop, noop, (c) =>
+      c.json({ success: true })
+    )
+    type Actual = ExtractSchema<typeof route>['/x9-arr']['$get']['output']
+    type Expected = { success: true }
+    type verify = Expect<Equal<Expected, Actual>>
+  })
+
+  test('app.on(method[], path, ...10 handlers) - last handler response type should be inferred', () => {
+    const noop: MiddlewareHandler = async (_c, next) => {
+      await next()
+    }
+    const route = app.on(
+      ['GET'],
+      '/x10-arr',
+      noop,
+      noop,
+      noop,
+      noop,
+      noop,
+      noop,
+      noop,
+      noop,
+      noop,
+      (c) => c.json({ success: true })
+    )
+    type Actual = ExtractSchema<typeof route>['/x10-arr']['$get']['output']
+    type Expected = { success: true }
+    type verify = Expect<Equal<Expected, Actual>>
+  })
 })
 
 describe('TypedResponse', () => {

src/types.ts

@@ -1790,7 +1790,7 @@ export interface OnHandlerInterface<
     ]
   ): HonoBase<
     IntersectNonAnyTypes<[E, E2, E3, E4, E5, E6, E7, E8, E9, E10, E11]>,
-    S & ToSchema<M, MergePath<BasePath, P>, I10, MergeTypedResponse<HandlerResponse<any>>>,
+    S & ToSchema<M, MergePath<BasePath, P>, I10, MergeTypedResponse<R>>,
     BasePath,
     MergePath<BasePath, P>
   >
@@ -2095,7 +2095,7 @@ export interface OnHandlerInterface<
     ]
   ): HonoBase<
     IntersectNonAnyTypes<[E, E2, E3, E4, E5, E6, E7, E8, E9, E10]>,
-    S & ToSchema<M, MergePath<BasePath, P>, I9, MergeTypedResponse<HandlerResponse<any>>>,
+    S & ToSchema<M, MergePath<BasePath, P>, I9, MergeTypedResponse<R>>,
     BasePath,
     MergePath<BasePath, P>
   >
@@ -2143,7 +2143,7 @@ export interface OnHandlerInterface<
     ]
   ): HonoBase<
     IntersectNonAnyTypes<[E, E2, E3, E4, E5, E6, E7, E8, E9, E10, E11]>,
-    S & ToSchema<M, MergePath<BasePath, P>, I10, MergeTypedResponse<HandlerResponse<any>>>,
+    S & ToSchema<M, MergePath<BasePath, P>, I10, MergeTypedResponse<R>>,
     BasePath,
     MergePath<BasePath, P>
   >