Incorrect error message on export assignment in `.d.ts` file

TypeScript Version: 3.5.3

Search Terms:

An export assignment cannot be used in a namespace.

Code

This is pretty niche, but I think it has general implications...

The current /lib/typescript.d.ts file is modelled as essentially a CommonJS file where at the end of the file, it has export = ts; which exports the whole namespace. The actual /lib/typescript.js though will place itself in the global scope as ts if it isn't loaded as a CommonJS module. In order to express that ts is available as a global variable, it requires code like this to be placed in a .d.ts file:

import * as _ts from "typescript";

declare global {
  namespace ts {
    export = _ts;
  }
}

Other methods of expressing this, like declare var ts: typeof _ts; only make the function shape of ts available, while the interfaces and type aliases and other type information is not accessible.

Expected behavior:

No errors.

Actual behavior:

The error TS1063: An export assignment cannot be used in a namespace occurs, but in VSCode, if the file is named globals.d.ts this error does not occur. 🤷‍♂ In all cases the code above does work like expected, making both the functional and type exports of the namespace available for code to be checked against.

首先使用提供的声明片段复现该诊断，然后将其与文件命名为 globals.d.ts 时的行为进行比较。阅读 /lib/typescript.d.ts 中相关的声明，以及 /lib/typescript.js 中的运行时上下文。当有效模式不报告错误，同时保留功能导出和类型导出时，即表示完成。