When providing identity function to map, it's return type is incorrect(see x2), while providing anonymous function as a callback leads to a correct output type(see x1).

TypeScript Version: 2.4.0 / nightly (2.5.0-dev.201xxxxx)

Code

const map = <T, U>(f: (x: T) => U, arr: T[]): U[] => {
  return arr.map((val) => f(val));
}

// x1 type is number[]
const x1 = map(x => x, [1, 2, 3]);

const identity = <T>(x: T) => x
// x2 type is {}[]
const x2 = map(identity, [1, 2, 3]);

Expected behavior:
Return type of map function, when providing identity function should be number[]

Actual behavior:
Return type is {}[]

Also, I've realized that map function in 2nd example does not infers even T type. It's {} currently, while it's obvious should be a number type, as we are providing array of numbers:

The second interesting thing is, that type inferring works fine, if map function arguments will be reversed:

const map = <T, U>(arr: T[], f: (x: T) => U): U[] => {
  return arr.map((val) => f(val));
}
// x2 type is number[]
const x2 = map1([1, 2, 3], identity);