Rationale (August, 2011):

The requirements apply to definitions, not declarations.

A constexpr function is required to have literal argument and return types. Consider an example like:

  template <class T> struct B {
    constexpr B(T) { }
  };

  struct A {
    B<A> b;
  };

Whether B(A) is constexpr depends on whether A is literal, which depends on whether B<A> is literal, which depends on whether B(A) is constexpr.