Update when we can infer `void` from a function body

[lucacasonato]

Right now we sometimes incorrectly infer the function return type due to our return statement analysis. The only cases where we can safely infer void is when:

• we are in a function declaration, there are no returns with value
• or when we are in a function expression or arrow function expression and either:
  • a single return statement without value is anywhere in the body
  • the body is empty

Notably, we can not infer a function expression from expressions such as () => { n() }, because the return type may be never if n() returns never.

Sync functions: https://www.typescriptlang.org/play/?#code/CYUwxgNghgTiAEAzArgOzAFwJYHtXxAA8sMAKASgC55UQA3EGAbgCgWiAHHGDJNTXPigV4AbxbxJBYmXKsAvm07deKdNjzwARiPFT4cDMhioWi9oS48+6wfDC6JUrInikA5hBxaoEACoAFlgAzuRiTvqSRCQUrPqK+obGpubK1moCmsCO+mZKlir2eMG8CAC8bmFlAHzh+tGyCvlWvGDFqvAVIjV1UkkmeRYtRagl8O6dlZ21es6uHl4+-kGhvZHSMXIR8Al9IEYDqQXWbaO8AZPdM9uDaa3t8FiTGRqoOVINsbfH92fwAFbPfivd6SfopZqFU5jADWQNseFBj3mnm8vkCITCs3Wny28W24O+w2hvAg8MybyxN3kQA Async functions: https://www.typescriptlang.org/play/?#code/CYUwxgNghgTiAEAzArgOzAFwJYHtXxAA8sMAKASgC55UQA3EGAbgCgWiAHHGDeKAZwCe6JGky58UCvADeLeAoLEy5VgF82nbrwHCwo9NjzwARtLmL4cDMhioWG9oS48+QkSkMT4Yc-MVYiPCkAOYQOCZQEAAqABZY-OSy-pYKRCQUrJYalta29o5arroeYkb4wH6WDprO2j54-LwIALxuesFJLQB8yZbpKuq1LrxgjbxBbSX60j19inl2NU4jDahN8CHwU+4zXb0WAUGh4ZEx8YnzqUoZqinwOQsgNkuFda5j67yx2+0iswd7ssiqNxvAsL9pgZxHgqooBplge9QV94AArSG7aHlOEKRYFYb1T4bADWmI6nhhqFx4OOYQiUTiCSSh2uCLu2Xu+KRq2JvAg5NKXlhLLYaiAA

[lucacasonato]

And classes and object literals: https://www.typescriptlang.org/play/?#code/CYUwxgNghgTiAEAzArgOzAFwJYHtXxAA8sMAKASgC55UQA3EGAbgCg2iAHHGDeSKAM4D4AQXgBvFvGnwoFCVJlKiJCqyUBfRdO3wARvMlKlcDMhipdWrS07deYPAN4AheAF4FSueS-HpKmTk6jIaADRsSga+Rv7wpuaWmiw2djx8TrwAwh7S-EJ+0j6FxoFqVpEy0SUmIGYWFRpAA

[marvinhagemeister]

For completeness: This requires to traverse into every block-like scope to potentially find return types:

export function foo() {
  for (let i = 0; i < 10; i++) {
    if (i === 10) {
      return 42
    }
  }
}

[lucacasonato]

We already do that 👍