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+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.