| 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. |