first draft of updates

Fill in the sections noted in the description of this PR.

Author: BillWagner

standard/classes.md

@@ -3942,9 +3942,11 @@ An abstract property declaration specifies that the accessors of the property ar
 
 A property declaration that includes both the `abstract` and `override` modifiers specifies that the property is abstract and overrides a base property. The accessors of such a property are also abstract.
 
-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.
+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 or interface 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. 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.
+For an override property `P` declared in an interface `I`, the overridden base property is determined by examining each direct or indirect base interface of `I`, collecting the set of interfaces declaring an accessible property which has the same name as `P`. 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 property declaration, then that is the overridden base 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. For an override property declared in an interface, there shall also 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 property and the type of every override of the overridden base property that is declared in a (direct or indirect) base interface of the overriding 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.
 

standard/expressions.md

@@ -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 `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 `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 specific ([§19.4.10](interfaces.md#19410-most-specific-implementation)) 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. 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.
+- 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 specific ([§19.4.10](interfaces.md#19410-most-specific-implementation)) 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
 
@@ -2358,6 +2358,13 @@ The binding-time processing of an indexer access of the form `P[A]`, where `P` i
   - If the indexer access is the target of an assignment then the indexer shall have a set or ref get accessor, otherwise, a binding-time error occurs;
   - Otherwise the indexer shall have a get or ref get accessor, otherwise a binding-time error occurs.
 
+The result of the indexer access is classified as follows:
+
+- If the indexer has a ref get accessor, the result is a variable with an associated type of the indexer type.
+- Otherwise, the result is a value with an associated type of the indexer type.
+
+If `T` is a class type, the associated type is picked from the first declaration or override of the indexer 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 indexer found in the most specific ([§19.4.10](interfaces.md#19410-most-specific-implementation)) of `T` or its direct or indirect base interfaces. It is a compile-time error if no unique such type exists.
+
 The runtime processing of the indexer access consists of the following steps:
 
 - The target *primary_expression* `P` is evaluated.

standard/interfaces.md

@@ -892,6 +892,8 @@ A *type_parameter_constraints_clause* on an explicit interface method implementa
 
 For an explicit interface member implementation to be valid, the class, struct, or interface shall name an interface in its base class or base interface list that contains a member whose qualified interface member name, type, number of type parameters, and parameter types exactly match those of the explicit interface member implementation. If an interface function member has a parameter array, the corresponding parameter of an associated explicit interface member implementation is allowed, but not required, to have the `params` modifier. If the interface function member does not have a parameter array then an associated explicit interface member implementation shall not have a parameter array.
 
+For an explicit interface member implementation of a method, property, or indexer that has a return type, there shall be an identity conversion or an implicit reference conversion from the return type of the explicit interface member implementation to the return type of every override of the interface member that is declared in a (direct or indirect) base interface.
+
 > *Example*: Thus, in the following class
 >
 > <!-- Example: {template:"standalone-lib", name:"ExplicitInterfaceMemberImplementations3", replaceEllipsis:true, customEllipsisReplacements:["return default;","return default;"], expectedErrors:["CS0540"]} -->
@@ -1104,6 +1106,8 @@ For purposes of interface mapping, a class, interface, or struct member `A` matc
 - `A` and `B` are events, and the name and type of `A` and `B` are identical.
 - `A` and `B` are indexers, the parameter lists of `A` and `B` are identical, `A` has the same accessors as `B` (`A` is permitted to have additional accessors if it is not an explicit interface member implementation), and the type of `A` is convertible to the return type of `B` via an identity conversion or, if `A` is a readonly indexer, an implicit reference conversion.
 
+> *Note*: Prior to the introduction of covariant return types, matching required an identity conversion between the return type of `A` and the return type of `B`. Allowing implicit reference conversions changes which member is selected as the implementation when a derived class re-implements an interface with a method that has a more derived return type. *end note*
+
 Notable implications of the interface-mapping algorithm are:
 
 - Explicit interface member implementations take precedence over other members in the same class or struct when determining the class or struct member that implements an interface member.