define in contract according to the caller, not the callee.

[deadalnix]

Everything start from this bug report : 
http://d.puremagic.com/issues/show_bug.cgi?id=6856

And the piece of code associated :

import std.stdio;

class A{
     void foo()in{writeln("in!");}out{writeln("out!");}body{}
}
class B:A{
     override void foo(){} // add in{assert(false);}body to get it working
}

void main(){
     A x = new A;
     x.foo(); // in! \ out!
     B y = new B;
     y.foo(); // out!
}

B.foo in contract isn't specified, so it is implicitly deduced that no 
preconditions have to be checked. As B.foo preconditions are always 
respected, A.foo precondition is never called on object of type B.

Now, let consider this function :

void fun(A a) {
     a.foo();
}

We can call fun with parameters of type A and B, because B is a subclass 
of A. If we call fun with a parameter of type B, then A.foo's 
precondition are not checked. This is a problem, because fun isn't 
supposed to know that it manipulate an object of type B, so should 
respect A.foo's preconditions. In fact, if fun call foo in a way that 
respect B.foo's precondition, but not A.foo's, it must be considered as 
an issue, because fun isn't supposed to know that it manipulate an 
object of type B.

Precondition are supposed to be respected by the caller, as opposed to 
postconditions. So the checked contract must be selected according to 
the type at caller's place, not callee's place.

In this regard, the bug report quoted here should be marked as invalid, 
but another open, with the restated issue. Or am I wrong ?