fix: allow Data to be inferred from Handler function (#2916)

This PR adds support for single-function GET handler data to be inferred
when using `define.page<typeof handler>()`.

```ts
export const handler = define.handlers((ctx) => {
  return page({ foo: "foo" });
});

export default define.page<typeof handler>((ctx) => {
  ctx.data; // Correctly resolves as { foo: string }
});
```

This PR also adds a test that checks that the types inferred are correct
by type checking.

Author: csvn

src/define.ts

@@ -1,5 +1,5 @@
 import type { AnyComponent } from "preact";
-import type { HandlerByMethod, RouteHandler } from "./handlers.ts";
+import type { HandlerByMethod, HandlerFn, RouteHandler } from "./handlers.ts";
 import type { Middleware } from "./middlewares/mod.ts";
 import type { PageProps } from "./context.ts";
 
@@ -107,7 +107,9 @@ export interface Define<State> {
   page<
     // deno-lint-ignore no-explicit-any
     Handler extends RouteHandler<any, State> = never,
-    Data = Handler extends HandlerByMethod<infer Data, State> ? Data : never,
+    Data = Handler extends HandlerFn<infer Data, State> ? Data
+      : Handler extends HandlerByMethod<infer Data, State> ? Data
+      : never,
   >(render: AnyComponent<PageProps<Data, State>>): typeof render;
 
   /**

src/define_test.ts

@@ -0,0 +1,45 @@
+import { expect } from "@std/expect";
+import { createDefine } from "./define.ts";
+import { page } from "./handlers.ts";
+
+Deno.test.ignore("createDefine", () => {
+  const define = createDefine<{ foo: number }>();
+
+  // Testing the types
+  const handlerFn = define.handlers((ctx) => {
+    ctx.state.foo satisfies number;
+    return page({ bar: true });
+  });
+  const handlerObj = define.handlers({
+    GET(ctx) {
+      ctx.state.foo satisfies number;
+      return page({ bar: [1, 2, 3] });
+    },
+    POST: () => {
+      return page({ baz: "hello" });
+    },
+  });
+
+  define.page<typeof handlerFn>(({ data }) => {
+    data.bar satisfies boolean;
+    return "page";
+  });
+
+  define.page<typeof handlerObj>(({ data }) => {
+    if ("baz" in data) {
+      data.baz satisfies string;
+    } else {
+      data.bar satisfies number[];
+    }
+    return "page";
+  });
+
+  define.middleware((ctx) => {
+    ctx.state.foo satisfies number;
+    return new Response("Hello");
+  });
+
+  expect(typeof define.page).toBe("function");
+  expect(typeof define.handlers).toBe("function");
+  expect(typeof define.middleware).toBe("function");
+});