// The method String.len is not a valid key selector function, but it does have
// a name, so it gets mentioned in the error context.
let f = "ABC".len;
[1, 2, 3].group_by(f)

# output:
stdin:4:20
  ╷
4 │ [1, 2, 3].group_by(f)
  ╵                    ^
Error: Unexpected argument. 'String.len' takes 0 arguments, but got 1.

stdin:4:20
  ╷
4 │ [1, 2, 3].group_by(f)
  ╵                    ^
In internal call to key selector from 'List.group_by'.

stdin:4:19
  ╷
4 │ [1, 2, 3].group_by(f)
  ╵                   ^
In call to method 'List.group_by'.