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:
#![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.