Missing error for `isolatedModules: true` and `export import`

[evanw]

Bug Report

🔎 Search Terms

isolatedModules true export import type-only namespace

🕗 Version & Regression Information

Version: typescript@4.3.5. This is the behavior in every version I tried.

⏯ Playground Link

N/A since this is a multi-file issue.

💻 Code

• jsx.ts

  export namespace JSXInternal {
   export type HTMLAttributes = string
   export type ComponentChildren = string
  }

• factory.ts

  import { JSXInternal } from "./jsx"
  
  export import JSX = JSXInternal;
  
  export function createElement<ElementType extends HTMLElement>(
   tagName: string,
   attributes: JSX.HTMLAttributes,
   ...children: JSX.ComponentChildren[]
  ): any {
   //...
  }

• tsconfig.json

  {
   "compilerOptions": {
   "target": "ES2015",
   "isolatedModules": true
   }
  }

🙁 Actual behavior

Repro:

1. Run tsc

2. Inspect factory.js, which looks like this:

   export function createElement(tagName, attributes, ...children) {
    //...
   }

3. Add export let foo to the JSXInternal namespace in jsx.ts

4. Run tsc again

5. Inspect factory.js, which now looks like this:

   import { JSXInternal } from "./jsx";
   export var JSX = JSXInternal;
   export function createElement(tagName, attributes, ...children) {
    //...
   }

🙂 Expected behavior

Since isolatedModules: true is enabled, I did not expect changing an input file to change the output of an unrelated output file. So I guess I expected for this input code to be considered a compile error under isolatedModules: true. I looked around a bit and couldn't find an already-filed issue about this so I filed this issue.

For context, I'm maintaining my own TypeScript parser for esbuild and I'm relying on isolatedModules: true to avoid cross-file compilation dependencies. My understanding is that's what isolatedModules: true is supposed to do (according to TypeScript's own documentation). Indeed TypeScript's ts.transpileModule API also miscompiles this code. This specific problem was brought to my attention by one of my users: evanw/esbuild#1550. Given the definition of isolatedModules, I consider this to be a bug in TypeScript's type checker which should be considering this code an error.

[andrewbranch]

Simple playground repro: https://www.typescriptlang.org/play?isolatedModules=true#code/PTAEAEEsGcHsBsCGAXApgEwLK3QV3qtAFyjIBOuqAUFZALYAOsZyoAZtO2bHaAEQc+NVAA8mLUPXGsA6mUhoAYpAIB5BskiwAdpwC87aADo5C1MrUatugNw0QoAG7GqDqc1nylK1Os079QxMvcx8-a2g7B1FpUABvUFNvS39dUABfOyA