This is just another case where prvalue is implied, like with the operands of almost every other operator.

Here is how I parse it:

For a call to a non-member function or to a static member function, the postfix expression shall either be:
- an
 lvalue that refers to a function (in which case the function-to-pointer standard conversion is suppressed on the postfix expression), or
- [a prvalue and] have function pointer type.