Allow default values for generic parameters

[israellot]

public T MethodWithGenericParamAndDefault<T>(T a, T b = default) { return a; }
The above method throws when parameter b is not set. This PR does a best effort in searching for a previously specified parameter of the same type to construct the default expression for this case.

When invoking MethodWithGenericParamAndDefault(1) it uses the knowledge the param a has the same type int as b.

[metoule]

It looks good, but I'd add the following test to be sure:

public List<T> MethodWithGenericParamAndDefault<T>(T a, List<T> b = default)
{
   return b;
}

var b = MethodWithGenericParamAndDefault(1);
Assert.Null(b);

[metoule]

LGTM! Thanks!

[davideicardi]

Thank you!