[Version 9.0] Feature support for covariant return types

standard/classes.md

@@ -3,28 +3,28 @@
 ## 15.1 General
 
 A class is a data structure that may contain data members (constants and fields), function members (methods, properties, events, indexers, operators, instance constructors, finalizers, and static constructors), and nested types. Class types support inheritance, a mechanism whereby a ***derived class*** can extend and specialize a ***base class***.
 
 Structs ([§16](structs.md#16-structs)) and interfaces ([§19](interfaces.md#19-interfaces)) have members similar to classes but with certain restrictions. This clause defines the declarations for classes and class members. The clauses for structs and interfaces define the restrictions for those types in terms of the corresponding declarations in class types.
 
 ## 15.2 Class declarations
 
 ### 15.2.1 General
 
 A *class_declaration* is a *type_declaration* ([§14.7](namespaces.md#147-type-declarations)) that declares a new class.
 
 ```ANTLR
 class_declaration
     : attributes? class_modifier* 'partial'? 'class' identifier
         type_parameter_list? class_base? type_parameter_constraints_clause*
         class_body ';'?
     ;
 ```
 
 A *class_declaration* consists of an optional set of *attributes* ([§23](attributes.md#23-attributes)), followed by an optional set of *class_modifier*s ([§15.2.2](classes.md#1522-class-modifiers)), followed by an optional `partial` modifier ([§15.2.7](classes.md#1527-partial-type-declarations)), followed by the keyword `class` and an *identifier* that names the class, followed by an optional *type_parameter_list* ([§15.2.3](classes.md#1523-type-parameters)), followed by an optional *class_base* specification ([§15.2.4](classes.md#1524-class-base-specification)), followed by an optional set of *type_parameter_constraints_clause*s ([§15.2.5](classes.md#1525-type-parameter-constraints)), followed by a *class_body* ([§15.2.6](classes.md#1526-class-body)), optionally followed by a semicolon.
 
 A class declaration shall not supply *type_parameter_constraints_clause*s unless it also supplies a *type_parameter_list*.
 
 A class declaration that supplies a *type_parameter_list* is a generic class declaration. Additionally, any class nested inside a generic class declaration or a generic struct declaration is itself a generic class declaration, since type arguments for the containing type shall be supplied to create a constructed type ([§8.4](types.md#84-constructed-types)).
 
 ### 15.2.2 Class modifiers
 
@@ -2747,6 +2747,8 @@
 
 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:
@@ -2755,8 +2757,11 @@
 - 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) 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.
@@ -3940,11 +3945,15 @@
 
 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.
+
+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.
 
-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.
+> *Note*: The accessibility constraint on the overriding property's type permits an override property in a `private` class to have a `private` property type. However, it requires a `public` override property in a `public` type to have a `public` property type. *end note*
 
-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.
+An overriding property declaration may include the `sealed` modifier. Use of this modifier prevents a derived class or interface from further overriding the property. The accessors of a sealed property are also sealed.
 
 Except for differences in declaration and invocation syntax, virtual, sealed, override, and abstract accessors behave exactly like virtual, sealed, override and abstract methods. Specifically, the rules described in [§15.6.4](classes.md#1564-virtual-methods), [§15.6.5](classes.md#1565-override-methods), [§15.6.6](classes.md#1566-sealed-methods), and [§15.6.7](classes.md#1567-abstract-methods) apply as if accessors were methods of a corresponding form:
 

standard/expressions.md

@@ -1,15 +1,15 @@
 # 12 Expressions
 
 ## 12.1 General
 
 An expression is a sequence of operators and operands. This clause defines the syntax, order of evaluation of operands and operators, and meaning of expressions.
 
 ## 12.2 Expression classifications
 
 ### 12.2.1 General
 
 The result of an expression is classified as one of the following:
 
 - A value. Every value has an associated type.
 - A variable. Unless otherwise specified, a variable is explicitly typed and has an associated type, namely the declared type of the variable. An implicitly typed variable has no associated type.
 - A null literal. An expression with this classification can be implicitly converted to a reference type or nullable value type.
@@ -583,7 +583,7 @@
 
 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 interface among the static type of the receiver and its direct and indirect base interfaces that contains a declaration or override of the function member. It is a compile-time error if no unique such most derived interface 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.
 
@@ -1693,7 +1693,7 @@
 - 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 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`.
@@ -1974,8 +1974,8 @@
 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, 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. 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 of the method found in the most specific (§19.4.10) 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
 
@@ -2334,6 +2334,13 @@
   - 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)) interface among `T` and 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

@@ -1,6 +1,6 @@
 # 19 Interfaces
 
 ## 19.1 General
 
 An interface defines a contract. A class or struct that implements an interface shall adhere to its contract. An interface may inherit from multiple base interfaces, and a class or struct may implement multiple interfaces.
 
@@ -892,6 +892,8 @@
 
 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 (if the member has a value return) 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"]} -->
@@ -1102,7 +1104,7 @@
 - `A` and `B` are methods, and the name, type, and parameter lists of `A` and `B` are identical.
 - `A` and `B` are properties, the name and type of `A` and `B` are identical, and `A` has the same accessors as `B` (`A` is permitted to have additional accessors if it is not an explicit interface member implementation).
 - `A` and `B` are events, and the name and type of `A` and `B` are identical.
-- `A` and `B` are indexers, the type and parameter lists of `A` and `B` are identical, and `A` has the same accessors as `B` (`A` is permitted to have additional accessors if it is not an explicit interface member implementation).
+- `A` and `B` are indexers, the type and parameter lists of `A` and `B` are identical, and `A` has the same accessors as `B` (`A` is permitted to have additional accessors if it is not an explicit interface member implementation).
 
 Notable implications of the interface-mapping algorithm are: