Why no implicit cast operators?

[jerro]

> E.g if I wanted to make a SafeInt struct, that behaves otherwise
> just like an int, but when operators like +=, *=, ++, -- etc are
> used with it, my custom SafeInt operators would check that
> there's no integer overflow.

> If you use alias this to _reveal_
> the internal int value of SafeInt, then that int value's default
> operators are used, and thus no overflow checking.

Wouldn't you have the exact same problem with implicit casts in 
C++? If you want to use custom operators, you should just define 
those, same as you would in C++. You can even implement just some 
operators and use alias this for functionality that you don't 
want to reimplement, like this:

import std.stdio;

struct A
{
     int a;
     alias a this;
     auto opOpAssign(string op, T)(T b)
         if(op == "+" && is(typeof(a += T.init)))
     {
         a += b;
         writeln("opOpAssign!\"+\" called.");
     }
}


void main()
{
     auto a = A(3);
     a += 5u;
     writeln(a);
     writeln(a - 1);
}

This prints:

opOpAssign!"+" called.
8
7