[ast-optimizer-branch] simplifying C++

[Jason Merrill]

I've been thinking about simplifying C++, and the complex scoping that C++
loops have.  In a for loop, a decl in the for-init-stmt is in scope for the
duration of the loop; a decl in the condition is in scope within an
iteration, goes out of scope, and back in.  The first can be dealt with
simply by wrapping the loop in a scope.  The second is more complex; I
think the best way to deal with it is to move the test into the loop body.

So given

  struct A {
    A(bool);
    operator bool();
    ~A();
  };

this

  for (int i = 42; A a (i == 24); --i)
    {
      ...
    }

becomes this pseudocode:

  {
    int i;
    for (i = 42; ; --i)
      {
        A* T.1;
        bool T.2;
        bool T.3;
        A a;
        T.1 = &a;
        T.2 = i == 24;
        A::ctor (T.1, T.2);
        __with_cleanup (A::dtor (T.1))
          { 
            T.3 = a.operator bool();
            if (T.3 == 0)
              break;
            ...
          }
      }
  }

I think it's best to leave the cleanup in this structured form initially,
and deal with adding it to the various block exits later.

Make sense?

Jason