Re: Type completeness as library requirement

According to N3126 - 3.9/9,

   "Scalar types, trivial class types (Clause 9), arrays of such types
and cv-qualified versions of these types (3.9.3) are collectively
called trivial types."

Thus, an array (possibly of unknown bound) can be trivial type, non-
trivial type, or an array type whose triviality cannot be determined
because its element type is incomplete.

According to N3126 - Table 45, preconditions for std::is_trivial are
defined as follows:

   "T shall be a complete type, (possibly cv-qualified) void, or an
array of unknown bound"

It seems that "an array of unknown bound" should be changed to "an
array of unknown bound of a complete element type". Preconditions for
some other templates (e.g., std::is_trivially_copyable,
std::is_standard_layout, std::is_pod, and std::is_literal_type) should
be changed similarly.

On the other hand, some preconditions look too restrictive. For
example, std::is_empty and std::is_polymorphic might accept any
incomplete non-class type.

Default allocator and containers like
std::vector might also be instantiated with incomplete element type:

     #include<vector>

     struct A;

     struct X
     {
         // what is the point to require
         // completely defined type A here?
         std::vector<A> v;
     };

     struct A
     {
         int i;
     };

     int main()
     {
         X x;
         x.v.push_back(A());
     }