Add covariant return types feature

Author: RexJaeschke

standard/classes.md

@@ -2746,6 +2746,8 @@ When an instance method declaration includes an `override` modifier, the method
 
 The method overridden by an override declaration is known as the ***overridden base method*** For an override method `M` declared in a class `C`, the overridden base method is determined by examining each base class of `C`, starting with the direct base class of `C` and continuing with each successive direct base class, until in a given base class type at least one accessible method is located which has the same signature as `M` after substitution of type arguments. For the purposes of locating the overridden base method, a method is considered accessible if it is `public`, if it is `protected`, if it is `protected internal`, or if it is either `internal` or `private protected`  and declared in the same program as `C`.
 
+For an override method `M` declared in an interface `I`, the overridden base method is determined by examining each direct or indirect base interface of `I`, collecting the set of interfaces declaring an accessible method which has the same signature as `M` after substitution of type arguments. If this set of interfaces has a *most derived type*, to which there is an identity or implicit reference conversion from every type in this set, and that type contains a unique such method declaration, then that is the overridden base method.
+
 The overriding method inherits any *type_parameter_constraints_clause*s of the overridden base method.
 
 A compile-time error occurs unless all of the following are true for an override declaration:
@@ -2754,8 +2756,11 @@ A compile-time error occurs unless all of the following are true for an override
 - There is exactly one such overridden base method. This restriction has effect only if the base class type is a constructed type where the substitution of type arguments makes the signature of two methods the same.
 - The overridden base method is a virtual, abstract, or override method. In other words, the overridden base method cannot be static or non-virtual.
 - The overridden base method is not a sealed method.
-- There is an identity conversion between the return type of the overridden base method and the override method.
+- The override method has a return type that is convertible by an identity conversion or (if the method has a value return) an implicit reference conversion to the return type of the overridden base method.
+- The override method has a return type that is convertible by an identity conversion or (if the method has a value return a) an implicit reference conversion to the return type of every override of the overridden base method that is declared in a (direct or indirect) base type of the override method.
 - The override declaration and the overridden base method have the same declared accessibility. In other words, an override declaration cannot change the accessibility of the virtual method. However, if the overridden base method is protected internal and it is declared in a different assembly than the assembly containing the override declaration then the override declaration’s declared accessibility shall be protected.
+- The override method's return type is at least as accessible as the override method.
+  > *Note*: This constraint permits an override method in a `private` class to have a `private` return type.  However, it requires a `public` override method in a `public` type to have a `public` return type. *end note*
 - A *type_parameter_constraints_clause* may only consist of the `class` or `struct` *primary_constraint*s applied to *type_parameter*s which are known according to the inherited constraints to be either reference or value types respectively. Any type of the form `T?` in the overriding method’s signature, where `T` is a type parameter, is interpreted as follows:
   - If a `class` constraint is added for type parameter `T` then `T?` is a nullable reference type; otherwise
   - If either there is no added constraint, or a `struct` constraint is added, for the type parameter `T` then `T?` is a nullable value type.
@@ -3939,7 +3944,7 @@ A property declaration that includes both the `abstract` and `override` modifier
 
 Abstract property declarations are only permitted in abstract classes ([§15.2.2.2](classes.md#15222-abstract-classes)) and interfaces ([§19.4.4](interfaces.md#1944-interface-properties)). The accessors of an inherited virtual property can be overridden in a derived class by including a property declaration that specifies an `override` directive. This is known as an ***overriding property declaration***. An overriding property declaration does not declare a new property. Instead, it simply specializes the implementations of the accessors of an existing virtual property.
 
-The override declaration and the overridden base property are required to have the same declared accessibility. In other words, an override declaration shall not change the accessibility of the base property. However, if the overridden base property is protected internal and it is declared in a different assembly than the assembly containing the override declaration then the override declaration’s declared accessibility shall be protected. If the inherited property has only a single accessor (i.e., if the inherited property is read-only or write-only), the overriding property shall include only that accessor. If the inherited property includes both accessors (i.e., if the inherited property is read-write), the overriding property can include either a single accessor or both accessors. There shall be an identity conversion between the type of the overriding and the inherited property.
+The override declaration and the overridden base property are required to have the same declared accessibility. In other words, an override declaration shall not change the accessibility of the base property. However, if the overridden base property is protected internal and it is declared in a different assembly than the assembly containing the override declaration then the override declaration’s declared accessibility shall be protected. The overriding property's type shall be at least as accessible as the overriding property. If the inherited property has only a single accessor (i.e., if the inherited property is read-only or write-only), the overriding property shall include only that accessor. If the inherited property includes both accessors (i.e., if the inherited property is read-write), the overriding property can include either a single accessor or both accessors. There shall be an identity conversion or (if the inherited property is read-only and has a value return) an implicit reference conversion between the type of the overriding and the inherited property.
 
 An overriding property declaration may include the `sealed` modifier. Use of this modifier prevents a derived class from further overriding the property. The accessors of a sealed property are also sealed.