[Version 9.0] Feature support for covariant return types

standard/classes.md

@@ -397,7 +397,7 @@
     : 'where' type_parameter ':' type_parameter_constraints
     ;
 
 type_parameter_constraints
     : primary_constraint (',' secondary_constraints)? (',' constructor_constraint)?
     | secondary_constraints (',' constructor_constraint)?
     | constructor_constraint
@@ -506,15 +506,15 @@
 >     static void M()
 >     {
 >         // nonnull constraint allows nonnullable struct type argument
 >         A<int> x1;
 >         // possible warning: nonnull constraint prohibits nullable struct type argument
 >         A<int?> x2;
 >         // nonnull constraint allows nonnullable class type argument
 >         A<C> x3;
 >         // possible warning: nonnull constraint prohibits nullable class type argument
 >         A<C?> x4;
 >         // nonnullable base class requirement allows nonnullable class type argument
 >         B1<C> x5;
 >         // possible warning: nonnullable base class requirement prohibits nullable class type argument
 >         B1<C?> x6;
 >         // nullable base class requirement allows nonnullable class type argument
@@ -2746,6 +2746,8 @@
 
 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:
@@ -2754,8 +2756,11 @@
 - 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 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.
@@ -3462,7 +3467,7 @@
 >     static void Main()
 >     {
 >         field = 10;
 >         Console.WriteLine(Property); // Prints 10
 >         Property = 20;               // This invokes the get accessor, then assigns
 >                                      // via the resulting variable reference
 >         Console.WriteLine(field);    // Prints 20
@@ -3939,7 +3944,7 @@
 
 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.
 
@@ -4577,7 +4582,7 @@
     : '!'
     ;
 
 overloadable_unary_operator
     : '+' | '-' | logical_negation_operator | '~' | '++' | '--' | 'true' | 'false'
     ;
 

standard/expressions.md

@@ -582,7 +582,7 @@
 
 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 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 @@
 - 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 @@
 
 - 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
 
@@ -2177,8 +2177,8 @@
 A *null_conditional_invocation_expression* is syntactically either a *null_conditional_member_access* ([§12.8.8](expressions.md#1288-null-conditional-member-access)) or *null_conditional_element_access* ([§12.8.13](expressions.md#12813-null-conditional-element-access)) where the final *dependent_access* is an invocation expression ([§12.8.10](expressions.md#12810-invocation-expressions)).
 
 A *null_conditional_invocation_expression* occurs within the context of a *statement_expression* ([§13.7](statements.md#137-expression-statements)), *anonymous_function_body* ([§12.21.1](expressions.md#12211-general)), or *method_body* ([§15.6.1](classes.md#1561-general)).
 
 Unlike the syntactically equivalent *null_conditional_member_access* or *null_conditional_element_access*, a *null_conditional_invocation_expression* may be classified as nothing.
 
 ```ANTLR
 null_conditional_invocation_expression
@@ -2258,7 +2258,7 @@
 - The *primary_expression* has compile-time type `dynamic`.
 - At least one expression of the *argument_list* has compile-time type `dynamic`.
 
 In this case the compile-time type of the *element_access* depends on the compile-time type of its *primary_expression*: if it has an array type then the compile-time type is the element type of that array type; otherwise the compile-time type is `dynamic` and the *element_access* is classified as a value of type `dynamic`. The rules below to determine the meaning of the *element_access* are then applied at run-time, using the run-time type instead of the compile-time type of those of the *primary_expression* and *argument_list* expressions which have the compile-time type `dynamic`. If the *primary_expression* does not have compile-time type `dynamic`, then the element access undergoes a limited compile-time check as described in [§12.6.5](expressions.md#1265-compile-time-checking-of-dynamic-member-invocation).
 
 > *Example*:
 >
@@ -3109,8 +3109,8 @@
 ### 12.8.18 The typeof operator
 
 The `typeof` operator is used to obtain the `System.Type` object for a type.
 
 ```ANTLR
 typeof_expression
     : 'typeof' '(' type ')'
     | 'typeof' '(' unbound_type_name ')'
@@ -3394,7 +3394,7 @@
 - one of the following value types: `sbyte`, `byte`, `short`, `ushort`, `int`, `uint`, `long`, `ulong`, `char`, `float`, `double`, `decimal`, `bool,`; or
 - any enumeration type.
 
 ### 12.8.22 Stack allocation
 
 A stack allocation expression allocates a block of memory from the execution stack. The ***execution stack*** is an area of memory where local variables are stored. The execution stack is not part of the managed heap. The memory used for local variable storage is automatically recovered when the current function returns.
 
@@ -6721,7 +6721,7 @@
 ## 12.23 Assignment operators
 
 ### 12.23.1 General
 
 All but one of the assignment operators assigns a new value to a variable, a property, an event, or an indexer element. The exception, `= ref`, assigns a variable reference ([§9.5](variables.md#95-variable-references)) to a reference variable ([§9.7](variables.md#97-reference-variables-and-returns)).
 
 ```ANTLR

standard/interfaces.md

@@ -507,7 +507,7 @@
 > <!-- Example: {template:"standalone-lib-without-using", name:"InterfaceNestedTypes", expectedErrors:["CS8427"]} -->
 > ```csharp
 > interface IOuter<out T>
 > {
 >     class C { } // error: class declaration within scope of variant type parameter 'T'
 > }
 > ```
@@ -1099,10 +1099,10 @@
 
 For purposes of interface mapping, a class, interface, or struct member `A` matches an interface member `B` when:
 
-- `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 methods, and the name, type, and parameter lists of `A` and `B` are identical, and the return type of `A` is convertible to the return type of `B` via an identity of implicit reference conversion to the return type of `B`.
+- `A` and `B` are properties, the name 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 property, an implicit reference conversion.
 - `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 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.
 
 Notable implications of the interface-mapping algorithm are: