C++11 decltype of member

Question

Why can't I do this:

class Foo {
  void fn();
  using fn_t = decltype(fn); //call to non-static member function without an object argument
};

But I can do

class Foo {
  static void fn();
  using fn_t = decltype(fn);
};

This SO post claims:

Within unevaluated operands (operands of decltype, sizeof, noexcept, ...) you can name nonstatic data members also outside of member functions

Answer 1

fn is a valid id-expression denoting a non-static member function. §5.1.1 [expr.prim.general]/p13 (footnote omitted):

An id-expression that denotes a non-static data member or non-static member function of a class can only be used:

• as part of a class member access (5.2.5) in which the object expression refers to the member's class or a class derived from that class, or
• to form a pointer to member (5.3.1), or
• if that id-expression denotes a non-static data member and it appears in an unevaluated operand.

§7.1.6.2 [dcl.type.simple]/p4:

The operand of the decltype specifier is an unevaluated operand (Clause 5).

Since decltype is not one of the few contexts in which an id-expression denoting a non-static member function may be used, the program is ill-formed.