Prerequisites

• I have read the Contributing Guidelines.
• I agree to follow the Code of Conduct.
• I have searched for existing issues that already report this problem, without success.

Stencil Version

4.29.2

Current Behavior

The type definition for the transform method of a Plugin incorrectly defines the return value as:

Promise<PluginTransformResults> | PluginTransformResults | string;

Expected Behavior

Based on the code the definitions should be something more like:

export interface Plugin {
  name?: string;
  pluginType?: string;
  load?: (id: string, context: PluginCtx) => Promise<string> | string;
  resolveId?: (importee: string, importer: string, context: PluginCtx) => Promise<string> | string;
  transform?: (
    sourceText: string,
    id: string,
    context: PluginCtx,
  ) => Promise<PluginTransformResults> | PluginTransformResults;
}

export type PluginTransformResults = PluginTransformationDescriptor | string | null;

export interface PluginTransformationDescriptor {
  code?: string;
  map?: string;
  id?: string;
  diagnostics?: Diagnostic[];
  dependencies?: string[];
}

System Info

System: node 20.18.0
    Platform: windows (10.0.26100)
   CPU Model: AMD Ryzen 9 9950X 16-Core Processor             (32 cpus)
    Compiler: C:\Users\user\Documents\GitHub\stencil-type-declaration-bug\node_modules\@stencil\core\compiler\stencil.js  
       Build: 1744045416
     Stencil: 4.29.2
  TypeScript: 5.5.4
      Rollup: 4.34.9
      Parse5: 7.2.1
      jQuery: 4.0.0-pre
      Terser: 5.37.0

Steps to Reproduce

1. Create a plugin with a transform method that returns null
2. Observe the Typescript language server getting mad at you

Code Reproduction URL

https://github.com/Altioc/stencil-type-declaration-bug/blob/ab78c5286d8e002fbe44048a85b6e2f02b444d26/stencil.config.ts#L7

Additional Information

I am willing to make a PR addressing this issue assuming it's an issue that the maintainers think needs addressing and that they think needs addressing in this way.