[c++0x] cannot return a constant

[Roman Kononov]

This is related to http://gcc.gnu.org/ml/gcc/2010-11/msg00623.html

I write about it again because the following seems too bad:

$ cat test1.cc 
struct X {
  X()=default;
  X(X&&)=default;
  X(X const&)=delete;
  //some very large or non-copyable content
};

X test() {
  X const x={};
  {
    //a lot of code where I do not want to modify x [accidentally]
  }
  return x;
}

$ g++ -c -std=c++0x test1.cc 
test1.cc: In function 'X test()':
test1.cc:13:10: error: use of deleted function 'X::X(const X&)'
test1.cc:4:3: error: declared here

Another related example:

$ cat test2.cc 
struct U {
  U();
  U(U&&);
  U(U const&);
};

struct X {
  U const u;
  X()=default;
  X(X&&)=default;
  //100 other members
};

X test() {
  X a={};
  return a;
}

$ g++ -c -std=c++0x test2.cc 
test2.cc: In function 'X test()':
test2.cc:16:10: error: use of deleted function 'X::X(X&&)'
test2.cc:10:3: error: 'X::X(X&&)' is implicitly deleted because the default 
definition would be ill-formed:
test2.cc:10:3: error: non-static data member 'X::u' does not have a move 
constructor or trivial copy constructor

In both examples, g++0x forces removing "const" (in "X const x={};" and
"U const u;") or doing some other ugly things like const_cast. In
general, it is not good to discourage using "const" for a number of
reasons. Additionally, if I have to have constness, I need to add
additional clutter reducing readability.

Can nothing be done to preserve "const"? Does the Standard really make
this code invalid? Gcc v4.5 was friendlier in this respect.