Allow return type annotations on setters

Setters can return values, so TypeScript should support declaring return types on setters.

🔎 Search Terms

setter return type

🕗 Version & Regression Information

This is the behavior in every version I tried, and I reviewed the FAQ for relevant entries

⏯ Playground Link

• Without return type: https://www.typescriptlang.org/play/?#code/MYewdgzgLgBAZiEMC8MDeAoGMIFNYBGAhgE4AUA+gJTpbYwn4CuJYMA5I1C2wG5EAbJrnYBuOgF8ME8aEiw8UKLhIQAwkTAAlZqwBqg4RBQwA8gQBWuYFAB0Ac3ymA7mAAKJEAAcVUAJ4AIrgQwCQAll5QIOQIIAA0HMQk7FS2imQ0yFkcXDww-EIisuAQIAK4tgIg9mQABorKqjDAmgAkaA0q6po63PqGwQD8HOwwAFwwAETsUJMSDLp8AxC1VKJAA
• With return type: https://www.typescriptlang.org/play/?#code/MYewdgzgLgBAZiEMC8MDeAoGMIFNYBGAhgE4AUA+gJQBcOUJAlmAObpbYwn4CuJYMAOTcofAQDciAGx65BAbg4BfDEsWhIsPFCi4SEAMJEwAJV78AatNkQUMAPIEAVrmBQAdC3z2A7mAAKJCAADnpQAJ4AIrgQwEzBUCDkCCAANELEJIJU7tpkVCjIqMLmEtZy6uAQIFK47lIgLGQABtq6+jDAxgAkaG16hsZmopblEAD8QoIwdABEglCzSlylMJIyMc1U8kA

💻 Code

const foo = {
  set bar(_) {
    return 'return value';
  }
};
const settersCanReturnValues = Object.getOwnPropertyDescriptor(foo, 'bar').set() === 'return value';
console.log(`setters can${settersCanReturnValues? '' : "'t"} return values`);

Same code, with an explicit return type:

const foo = {
  set bar(_): string {
    return 'return value';
  }
};
const settersCanReturnValues = Object.getOwnPropertyDescriptor(foo, 'bar').set() === 'return value';
console.log(`setters can${settersCanReturnValues? '' : "'t"} return values`);

🙁 Actual behavior

TypeScript compiler emits invalid JavaScript containing return type declaration syntax.

🙂 Expected behavior

TypeScript compiler supports and removes return type declarations from setters.

Additional information about the issue

从两个 TypeScript Playground 示例开始，比较具有和不具有显式返回类型的 setter 所生成的 JavaScript。当带注解的 setter 能够编译为移除其返回类型的有效 JavaScript，同时现有的无注解示例继续正常工作时，此更改即完成；在 compiler 现有的测试套件中添加一个回归测试。