Generator helper should include [Symbol.toStringTag]

TypeScript Version: 3.8.0-dev.20191223 and 3.7.3

Search Terms:

Generator, toStringTag, transformer, helper, tslib

Code

The following code will error in when using compilerOptions.target = es5, but works correctly when compiled with target = es2015 or greater.

// index.ts
function* f() {}
if (f()[Symbol.toStringTag] !== 'Generator') {
	  throw new Error("Bad generator!")
}
console.log('👍 Works well!');

tsc --target es5 --lib dom,es5,es2015 ./index.ts && node index.js
# Error: Bad generator!
tsc --target es2015 ./index.ts && node index.js
# 👍Works well!

Expected behavior:

The property should be set as expected via the helpers.

Generators should include the Symbol.toStringTag property "Generator" according to spec.

Some libraries check for a generator by inspecting this property. For example, @wordpress/redux-routine checks for this property and does not correctly handle generators produced by the helper.

regenerator-runtime is a widely used generator helper which does set this property.

Actual behavior:

Symbol.toStringTag is not included on the generator.

Playground Link:

Broken with target=es5
Working with target=es2015

Related Issues:

#19006