Making a member virtual should be considered a breaking change

[MichalStrehovsky]

The breaking change document currently considers making a member virtual a non-breaking change with a small sidenote. When that rule was decided, the C# compiler was only emitting call IL instruction in a couple places that didn't really matter and used the IL callvirt for everything else (even if the destination instance method was non-virtual, because per the C# spec, calling instance methods with a null this needs to throw and call IL instruction won't throw). This made it unlikely that people would hit problems with it.

The addition of the null-propagation operator gave the C# compiler more opportunities to optimize callvirt into a call if the this is known to be not null.

dotnet/corert#3565 has an example where making a member virtual resulted in wrong method being called when compiling against PCL contracts because a breaking change was made making ConstructorInfo.Invoke virtual:

var abc = (ABC) constr.Invoke(new object[0]); // This works

var abc = (ABC) constr?.Invoke(new object[0]); // This calls the wrong method because
                                               // `this` is known not to be null and Invoke
                                               // is a non-virtual method

Making a member virtual is a breaking change for anyone using the null-propagation operator with the method.

[danmoseley]

Related to dotnet/corefx#18138

[karelz]

Next step: We need PR with right wording ...