Arrow functions treated differently than functions and methods

[aleclarson]

TypeScript Version: 3.2.2 and 3.3.0-dev.20190119

Search Terms: arrow function method object literal

Code

type Test<K extends keyof any> = {
  [P in K | "foo"]: P extends "foo" ? true : () => any
}

function test<T extends Test<keyof T>>(arg: T) {
    return arg;
}

const res1 = test({
  foo: true,
  bar() {
    // Notice the type param of `test` is inferred as `Test<never>`
  }
})

const res2 = test({
  foo: true,
  bar: function () {
    // Same error here.
  }
})

const res3 = test({
  foo: true,
  bar: () => {
    // This is fine.
  }
})

Expected behavior: The first two test calls should infer the given object type correctly.

Actual behavior: They instead infer the object type as Test<never>. 😢

Playground Link: Click here

Related Issues: #17867 (sort of)

• https://stackoverflow.com/questions/50198477/how-to-type-check-this-object-without-using-or-any

[aleclarson]

Interestingly, the following works:

const res4 = test({
  foo: true,
  bar,
})
function bar() {
  // This is fine.
}