Playground: Should support looping for generator

[lanshunfang]

TypeScript Version: 3.4.0-dev.201xxxxx

Search Terms:

• Generator
• Generator loop
• Generator for
• Generator iterator

Code

// A *self-contained* demonstration of the problem follows...
// Test this by running `tsc` on the command-line, rather than through another build tool such as Gulp, Webpack, etc.
const gen = function* () { 
    yield 2;
    yield 3;
}
const genInst = gen();
for (let i of genInst) {
    console.log(i);
}

Expected behavior:

• No error is thrown
• console.log should output yield value 2, 3
• Just try it in Chrome debugger

Actual behavior:

• TSC: throw type error
• error TS2569: Type 'IterableIterator<2 | 3>' is not an array type or a string type.

Playground Link:

• https://www.typescriptlang.org/play/#src=const%20gen%20%3D%20function*%20()%20%7B%20%0D%0A%20%20%20%20yield%202%3B%0D%0A%20%20%20%20yield%203%3B%0D%0A%7D%0D%0Aconst%20genInst%20%3D%20gen()%3B%0D%0Afor%20(let%20i%20of%20genInst)%20%7B%0D%0A%20%20%20%20alert(i)%0D%0A%7D

Related Issues:

• Playground: Add toggle to enable downlevelIteration flag #18209

[lanshunfang]

With --downlevelIteration in tsc (not playground, this problem solved)

[lanshunfang]

Duplicated with #18209