A project targeting ES3 that transitively depends on @types/acorn will error on compile #24595

• I tried using the @types/acorn package and had problems.
• I tried using the latest stable version of tsc. https://www.npmjs.com/package/typescript
• I have a question that is inappropriate for StackOverflow. (Please ask any appropriate questions there).
• Mention the authors (see Definitions by: in index.d.ts) so they can respond.
  • Authors: @RReverser, @e-cloud

Repro steps:

In a clean folder (not inside a repo with a node_modules/ already):

echo 'console.log(`hello world`);' > foo.ts
npm install --no-save @types/acorn
tsc foo.ts

Expected behavior: foo.ts compiles with no problem.

Observed behavior:

node_modules/@types/acorn/index.d.ts(240,8): error TS2304: Cannot find name 'Symbol'.
node_modules/@types/acorn/index.d.ts(240,28): error TS2304: Cannot find name 'Iterator'.

I tried to fix this, but the only thing that seems to make the error go away is:

declare global {
  interface SymbolConstructor {
    readonly iterator: symbol;
  }
  var Symbol: SymbolConstructor;
}

Which leaks Symbol and Symbol.iterator into the global scope for foo.ts, when it shouldn't be available since foo.ts is being compiled to target ES3.

I also tried looking into what other type packages do to fix this, but I only found other type packages that have a similar problem, for example @types/zen-observables:

node_modules/@types/zen-observable/index.d.ts(35,14): error TS2304: Cannot find name 'Symbol'.
node_modules/@types/zen-observable/index.d.ts(46,6): error TS2304: Cannot find name 'Symbol'.