| The built-in function traits are generic over a tuple of the function arguments. |
| If one uses angle-bracket notation (`Fn<(T,), Output=U>`) instead of parentheses |
| (`Fn(T) -> U`) to denote the function trait, the type parameter should be a |
| tuple. Otherwise function call notation cannot be used and the trait will not be |
| implemented by closures. |
| |
| The most likely source of this error is using angle-bracket notation without |
| wrapping the function argument type into a tuple, for example: |
| |
| ```compile_fail,E0059 |
| #![feature(unboxed_closures)] |
| |
| fn foo<F: Fn<i32>>(f: F) -> F::Output { f(3) } |
| ``` |
| |
| It can be fixed by adjusting the trait bound like this: |
| |
| ``` |
| #![feature(unboxed_closures)] |
| |
| fn foo<F: Fn<(i32,)>>(f: F) -> F::Output { f(3) } |
| ``` |
| |
| Note that `(T,)` always denotes the type of a 1-tuple containing an element of |
| type `T`. The comma is necessary for syntactic disambiguation. |
