Add covariant return types feature

Author: RexJaeschke

standard/expressions.md

@@ -582,7 +582,7 @@ For each argument in an argument list there has to be a corresponding parameter
 
 The parameter list used in the following is determined as follows:
 
-- For virtual methods and indexers defined in classes, the parameter list is picked from the first declaration or override of the function member found when starting with the static type of the receiver, and searching through its base classes.
+- For virtual methods and indexers defined in classes, the parameter list is picked from the first declaration or override of the function member found when starting with the static type of the receiver, and searching through its base classes. For virtual methods and indexers defined in interfaces, the parameter list is picked from the declaration or override of the function member found in the most derived type among those types containing the declaration of override of the function member. It is a compile-time error if no unique such type exists.
 - For partial methods, the parameter list of the defining partial method declaration is used.
 - For all other function members and delegates there is only a single parameter list, which is the one used.
 
@@ -1717,7 +1717,7 @@ The *member_access* is evaluated and classified as follows:
 - If `E` is a property access, indexer access, variable, or value, the type of which is `T`, and a member lookup ([§12.5](expressions.md#125-member-lookup)) of `I` in `T` with `K` type arguments produces a match, then `E.I` is evaluated and classified as follows:
   - First, if `E` is a property or indexer access, then the value of the property or indexer access is obtained ([§12.2.2](expressions.md#1222-values-of-expressions)) and E is reclassified as a value.
   - If `I` identifies one or more methods, then the result is a method group with an associated instance expression of `E`.
-  - If `I` identifies an instance property, then the result is a property access with an associated instance expression of `E` and an associated type that is the type of the property. If `T` is a class type, the associated type is picked from the first declaration or override of the property found when starting with `T`, and searching through its base classes.
+  - If `I` identifies an instance property, then the result is a property access with an associated instance expression of `E` and an associated type that is the type of the property. If `T` is a class type, the associated type is picked from the first declaration or override of the property found when starting with `T`, and searching through its base classes. If `T` is an interface type, the associated type is picked from the declaration or override of the property found in the most derived of `T` or its direct or indirect base interfaces. It is a compile-time error if no unique such type exists.
   - If `T` is a *class_type* and `I` identifies an instance field of that *class_type*:
     - If the value of `E` is `null`, then a `System.NullReferenceException` is thrown.
     - Otherwise, if the field is readonly and the reference occurs outside an instance constructor of the class in which the field is declared, then the result is a value, namely the value of the field `I` in the object referenced by `E`.
@@ -1999,7 +1999,7 @@ The result of evaluating an *invocation_expression* is classified as follows:
 
 - If the *invocation_expression* invokes a returns-no-value method ([§15.6.1](classes.md#1561-general)) or a returns-no-value delegate, the result is nothing. An expression that is classified as nothing is permitted only in the context of a *statement_expression* ([§13.7](statements.md#137-expression-statements)) or as the body of a *lambda_expression* ([§12.21](expressions.md#1221-anonymous-function-expressions)). Otherwise, a binding-time error occurs.
 - Otherwise, if the *invocation_expression* invokes a returns-by-ref method ([§15.6.1](classes.md#1561-general)) or a returns-by-ref delegate, the result is a variable with an associated type of the return type of the method or delegate. If the invocation is of an instance method, and the receiver is of a class type `T`, the associated type is picked from the first declaration or override of the method found when starting with `T` and searching through its base classes.
-- Otherwise, the *invocation_expression* invokes a returns-by-value method ([§15.6.1](classes.md#1561-general)) or returns-by-value delegate, and the result is a value, with an associated type of the return type of the method or delegate. If the invocation is of an instance method, and the receiver is of a class type `T`, the associated type is picked from the first declaration or override of the method found when starting with `T` and searching through its base classes.
+- Otherwise, the *invocation_expression* invokes a returns-by-value method ([§15.6.1](classes.md#1561-general)) or returns-by-value delegate, and the result is a value, with an associated type of the return type of the method or delegate. If the invocation is of an instance method, and the receiver is of a class type `T`, the associated type is picked from the first declaration or override of the method found when starting with `T` and searching through its base classes. If the invocation is of an instance method, and the receiver is of an interface type `T`, the associated type is picked from the declaration or override of the method found in the most derived interface from among `T` and its direct and indirect base interfaces. It is a compile-time error if no unique such type exists.
 
 #### 12.8.10.2 Method invocations