Function expressions widen to `{}` in auto arrays

🔎 Search Terms

function expression widen widening auto array any empty objecr

🕗 Version & Regression Information

• This is the behavior in every version I tried

⏯ Playground Link

https://www.typescriptlang.org/play?ts=5.5.0-dev.20240304#code/MYewdgzgLgBAhgJwQRhgXhgbQLoG4BQ+iKAdAA4CuEAFgBQBmFYwUAluDLQJQwDe+MGAgCmUCgjAwARFIIBfLgSJJkBAPRqYAPQD8hUJFjEATOix5CJ8lTrd0APj4Cho8ZJnzFlpMfWbd+EA

💻 Code

const arr1 = [];

arr1.push(function () {
  return "";
});

arr1;
// ^? const arr1: {}[]

const arr2 = [];

arr2.push(() => {
  return "";
});

arr2;
// ^? const arr2: (() => string)[]

🙁 Actual behavior

arr1's type after this .push is {}[]

🙂 Expected behavior

I'd expect the type at both of the checked locations to be the same ((() => string)[])

Additional information about the issue

Likely the difference comes from the fact that the function expression is context-sensitive - maybe this is just a design limitation. However, the same doesn't happen in JS with expando objects (TS playground):

const obj = {}

obj.foo = function () {
  return ""
}

const result = obj.foo
//    ^? const result: () => string

Start with the linked TypeScript Playground example and compare the inferred types of arr1 and arr2 after their respective push calls. Use the additional JavaScript expando-object example as a comparison point; done means the function-expression and arrow-function cases have consistent inferred array element types, with regression coverage for the reported behavior.