Provide way to configure default compiler options for TS Server Inferred Project

Search Terms

Inferred Project
TS Server

Suggestion

Add an option to configure the default compilerOptions for inferred projects outside of the host.

Related to: https://docs.google.com/document/d/1eB6cGCG_2ircfS5GzpDC9dBgikeYYcMxghVH5sDESHw/edit#heading=h.telvo0tzo7t0 and https://github.com/microsoft/TypeScript/issues/39632

Checklist

Use Cases

Currently, there is no way to configure the default compilerOptions for inferred projects outside of the server host. Being able to configure should options would be give a better DX when not using the default compiler options such as experimentalDecorators, strictNullChecks etc... together with stricter file inclusions, ie favoring explicitly listing the entry-point files using files instead of include with globbing.

Previously when not using TypeScript solution style tsconfig this could be achieved by adding a tsconfig.json in the root of the project which will include all files and set these options example.

{
   "compilerOptions":  {
     "experimentalDecorators": true,
     "strictNullChecks": true,
     ...
   }
}

This trickery cannot be used when using a solution style tsconfig, because the root tsconfig will not contain any files due to files: [] setting.

Examples

Ideally, compiler options for inferred projects are configured in a TypeScript configuration file. When using solution style tsconfig, such configuration can be set here.

{
 "files": [],
 "compilerOptions":  {
   "experimentalDecorators": true,
   "strictNullChecks": true,
   ...
 },
 "references": [
   {
     "path": "./tsconfig.app.json"
   },
   {
     "path": "./tsconfig.spec.json"
   }
 ]
}

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

//cc @kyliau

先从 TS Server 的 inferred projects 行为和关联的 issue 39632 入手，然后检查如何处理包含 files: [] 的 solution-style tsconfig 文件。确定为 compilerOptions 中的 experimentalDecorators 和 strictNullChecks 等默认值所需的配置位置和优先级。完成标准是 inferred projects 可以获得这些默认值，同时不改变现有项目的行为。