Contracts type narrowing

[piotr-lasota]

Scope

Types narrowing for IContract related registration and Contract<T>

#Justification

IContractRegistry

Current constraints allow for any class to act as a type parameter for .Register<T>() method. As this would not work I want to improve the developer experience by allowing the compiler to indicate that a given invocation is not supported

Contract<T>

Semantically contracts are intended to validate messages exchanged between modules. Narrowing down the type constraint helps to enforce those sematics.