Contract programing restrictions

[iackhtak via Digitalmars-d-learn]

There was discussion about broken contract programing. Broken 
thing was "in" contract within inheritance. If you add different 
"in"-contract in overridden parent and derived function only one 
will be checked.
I thought that solution is to ban "in"-contract for derived 
function. "In"-contract says what you can pass. If you want to 
accept input without any constraints you can't add new constraint 
in inherited stuff because your inherited stuff can by used in 
any context where base thing can. Within this context any 
arguments can be passed. This way adding new constraints on input 
brakes Liskov substitution principle and have to be banned.
Theoretically it can be allowed to loose contract(and extend 
interface) but the thing looks hard to implement.

Conversely "out" can be narrowed without any restrictions.