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2.1 breaks promise chaining #12886

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@adrienverge

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@adrienverge

This (apparently valid) code compiled OK with TypeScript 2.0:

function f(): Promise<void> {
  return Promise.resolve(4).then((n: number) => {
    if (n !== 200) {
      return Promise.reject('error');
    }
    return Promise.resolve();
  });
}

With TypeScript 2.1.4 it gives:

test.ts(2,10): error TS2322: Type 'Promise' is not assignable to type 'Promise'.
Type 'number' is not assignable to type 'void'.

tsconfig.json:

{
  "compilerOptions": {
    "target": "es5",
    "lib": ["es2015"]
  }
}

It seems that the problem comes when promises are chained. For instance when removing the .then() chain, compilation runs without any error:

function f(n: number): Promise<void> {
  if (n !== 200) {
    return Promise.reject('error');
  }
  return Promise.resolve();
}

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