Invalid type hint for onWarn(): defaultHandler parameter should not be optional

[BlueGreenMagick]

Describe the bug

Current type hint for onwarn is as follows:

interface SvelteConfig {
    onwarn?: (warning: Warning, defaultHandler?: (warning: Warning) => void) => void;
}

This type hint is wrong in that defaultHandler is marked optional. With this definition, when writing vite.config.ts with typescript strict flag enabled, the onwarn handler must handle the case of undefined being passed as defaultHandler argument.

const options = {
    //...
    onwarn: (warning, defaultHandler) => {
        defaultHandler(warning); // ERROR: `defaultHandler` is possibly 'undefined'.
    }
}

However, the defaultHandler is always passed to the onwarn handler.

vite-plugin-svelte/packages/vite-plugin-svelte/src/utils/log.js

Line 153 in acb1f88

onwarn(warning, warn);

The typescript error is because (1) (Warning, ((Warning) => void) | undefined) => void is a subtype of (2) (Warning, (Warning => void)) => void, not the other way around. So you can't pass (2) into a place that accepts (1).

Therefore, the correct type should be to remove the optional mark ? from defaultHandler. Such definition still accepts an onwarn function without defaultHandler parameter (warning) => void.
c.f. https://www.typescriptlang.org/docs/handbook/type-compatibility.html#comparing-two-functions
tested here: https://stackblitz.com/edit/vitejs-vite-86jcat?file=typescript.ts

Note that the bug only occurs when the typescript strictFunctionTypes flag, or strict flag is enabled. Without it, the type system is relaxed so that a supertype function type can go into a place that accepts subtype function type. (https://www.typescriptlang.org/docs/handbook/type-compatibility.html#function-parameter-bivariance)

Reproduction URL

https://stackblitz.com/edit/vitejs-vite-86jcat?file=vite.config.ts

Reproduction

1. Run tsc --noEmit in the terminal to typecheck using tsc

It throws an error that Cannot invoke an object which is possibly 'undefined'. for the line defaultHandler(warning) in vite.config.ts.

export default defineConfig({
  plugins: [
    svelte({
      onwarn: (warning, defaultHandler) => {
        defaultHandler(warning); // ERROR!
      },
    }),
  ],
});

Logs

No response

System Info

System:
    OS: Linux 5.0 undefined
    CPU: (8) x64 Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz
    Memory: 0 Bytes / 0 Bytes
    Shell: 1.0 - /bin/jsh
  Binaries:
    Node: 18.18.0 - /usr/local/bin/node
    Yarn: 1.22.19 - /usr/local/bin/yarn
    npm: 10.2.3 - /usr/local/bin/npm
    pnpm: 8.15.3 - /usr/local/bin/pnpm
  npmPackages:
    @sveltejs/vite-plugin-svelte: ^3.0.2 => 3.1.0 
    svelte: ^4.2.12 => 4.2.14 
    vite: ^5.2.8 => 5.2.8