[Version 9.0] Feature support for unconstrained type parameter annotations

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
 
@@ -409,6 +409,7 @@
     | 'struct'
     | 'notnull'
     | 'unmanaged'
+    | 'default'
     ;
 
 secondary_constraint
@@ -429,7 +430,11 @@
 
 The list of constraints given in a `where` clause can include any of the following components, in this order: a single primary constraint, one or more secondary constraints, and the constructor constraint, `new()`.
 
-A primary constraint can be a class type, the ***reference type constraint*** `class`, the ***value type constraint*** `struct`, the ***not null constraint*** `notnull` or the ***unmanaged type constraint*** `unmanaged`. The class type and the reference type constraint can include the *nullable_type_annotation*.
+A primary constraint can be a class type, the ***reference type constraint*** `class`, the ***value type constraint*** `struct`, the ***not null constraint*** `notnull`, the ***unmanaged type constraint*** `unmanaged`, or `default`. The class type and the reference type constraint can include the *nullable_type_annotation*.
+
+It is a compile-time error to use a `default` constraint other than on a method override or explicit interface member implementation. It is a compile-time error to use a `default` constraint when the corresponding type parameter in the overridden or interface method is constrained to a reference type or non-nullable value type.
+
+> *Note*: A class type constraint such as `where T : Stream` constrains the type parameter to a reference type; the `default` constraint may not be used with such a constraint. *end note*
 
 A secondary constraint can be an *interface_type* or *type_parameter*, optionally followed by a *nullable_type_annotation*. The presence of the *nullable_type_annotation* indicates that the type argument is allowed to be the nullable reference type that corresponds to a non-nullable reference type that satisfies the constraint.
 
@@ -444,12 +449,14 @@
 
 > *Note*: To specify that a type argument is a nullable reference type, do not add the nullable type annotation as a constraint (use `T : class` or `T : BaseClass`), but use `T?` throughout the generic declaration to indicate the corresponding nullable reference type for the type argument. *end note*
 
-<!-- Remove in C# 9, when `?` is allowed on any type parameter. -->
-The nullable type annotation, `?`, can only be used on a type parameter that has the value type constraint, the reference type constraint without the *nullable_type_annotation*, or a class type constraint without the *nullable_type_annotation*.
+Except when a type parameter is explicitly constrained to value types, the nullable type annotation `?` can only be applied to a type parameter when the nullable annotations flag is enabled ([§6.5.9](lexical-structure.md#659-nullable-directive)).
 
-<!-- Add in C# 9, when `?` is allowed on nullable reference type parameters. -->
-<!-- For a type parameter `T` when the type argument is a nullable reference type `C?`, instances of `T?` are interpreted as `C?`, not `C??`. -->
+For a type parameter `T` when the type argument is a nullable reference type `C?`, instances of `T?` are interpreted as `C?`, not `C??`.
 
+> *Note*: On a type parameter, a return type `T?` has the same nullability effect as `[MaybeNull] T`, and a parameter type `T?` has the same nullability effect as `[AllowNull] T`. *end note*
+<!-- markdownlint-disable MD028 -->
+
+<!-- markdownlint-enable MD028 -->
 > *Example*: The following examples show how the nullability of a type argument impacts the nullability of a declaration of its type parameter:
 >
 > <!-- Example: {template:"standalone-lib-without-using", name:"RepeatedNullable"} -->
@@ -534,6 +541,8 @@
 
 Because `unmanaged` is not a keyword, in *primary_constraint* the unmanaged constraint is always syntactically ambiguous with *class_type*. For compatibility reasons, if a name lookup ([§12.8.4](expressions.md#1284-simple-names)) of the name `unmanaged` succeeds it is treated as a `class_type`. Otherwise it is treated as the unmanaged constraint.
 
+The `default` constraint may be used for a type parameter whose inherited constraints do not establish it as a reference type or a value type; its effect on `T?` in override methods and explicit interface member implementations is specified in [§15.6.5](classes.md#1565-override-methods) and [§19.6.2](interfaces.md#1962-explicit-interface-member-implementations).
+
 Pointer types are never allowed to be type arguments, and do not satisfy any type constraints, even unmanaged, despite being unmanaged types.
 
 If a constraint is a class type, an interface type, or a type parameter, that type specifies a minimal “base type” that every type argument used for that type parameter shall support. Whenever a constructed type or generic method is used, the type argument is checked against the constraints on the type parameter at compile-time. The type argument supplied shall satisfy the conditions described in [§8.4.5](types.md#845-satisfying-constraints).
@@ -2165,7 +2174,7 @@
 
 A generic method is a method whose declaration includes a *type_parameter_list*. This specifies the type parameters for the method. The optional *type_parameter_constraints_clause*s specify the constraints for the type parameters.
 
-A generic *method_declaration*, either with an `override` modifier, or for an explicit interface member implementation, inherits type parameter constraints from the overridden method or interface member respectively. Such declarations may only have *type_parameter_constraints_clause*s containing the *primary_constraint*s `class` and `struct`, the meaning of which in this context is defined in [§15.6.5](classes.md#1565-override-methods) and [§19.6.2](interfaces.md#1962-explicit-interface-member-implementations) for overriding methods and explicit interface implementations respectively.
+A generic *method_declaration*, either with an `override` modifier, or for an explicit interface member implementation, inherits type parameter constraints from the overridden method or interface member respectively. Such declarations may only have *type_parameter_constraints_clause*s containing the *primary_constraint*s `class`, `struct`, and `default`, the meaning of which in this context is defined in [§15.6.5](classes.md#1565-override-methods) and [§19.6.2](interfaces.md#1962-explicit-interface-member-implementations) for overriding methods and explicit interface implementations respectively.
 
 The *member_name* specifies the name of the method. Unless the method is an explicit interface member implementation ([§19.6.2](interfaces.md#1962-explicit-interface-member-implementations)), the *member_name* is simply an *identifier*.
 
@@ -2757,9 +2766,13 @@
 - 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 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.
-- 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.
+- A *type_parameter_constraints_clause* may only consist of the `class`, `struct`, or `default` *primary_constraint*s. The `class` and `struct` constraints may be placed on *type_parameter*s known according to the inherited constraints to be either reference or non-nullable value types respectively. The `default` constraint may be placed on *type_parameter*s that are not constrained to either reference or value types. 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.
+  - If either a `struct` constraint is added, or no constraint is added and the inherited constraint is a value type constraint, for the type parameter `T` then `T?` is a nullable value type.
+  - If a `default` constraint is added for type parameter `T`, the interpretation of `T?` depends on the type argument substituted for `T`:
+    - If `T` is substituted with a reference type `C`, `T?` represents the nullable reference type `C?`.
+    - If `T` is substituted with an already-annotated reference type `C?`, `T?` represents `C?`, not `C??`.
+    - If `T` is substituted with a value type, `T?` represents `T` itself, not `Nullable<T>`.
 
 > *Example*: The following demonstrates how the overriding rules work for generic classes:
 >
@@ -2809,6 +2822,28 @@
 > ```
 >
 > Without the type parameter constraint `where T : class`, the base method with the reference-typed type parameter cannot be overridden. *end example*
+<!-- markdownlint-disable MD028 -->
+
+<!-- markdownlint-enable MD028 -->
+> *Example*: The following demonstrates how the `default` constraint works when overriding a method with an unconstrained type parameter:
+>
+> <!-- Example: {template:"standalone-lib-without-using", name:"OverrideMethods6"} -->
+> ```csharp
+> #nullable enable
+> class A2
+> {
+>     public virtual void F2<T>(T? t) where T : struct { }
+>     public virtual void F2<T>(T? t) { }
+> }
+>
+> class B2 : A2
+> {
+>     public override void F2<T>(T? t) /*where T : struct*/ { }
+>     public override void F2<T>(T? t) where T : default { }
+> }
+> ```
+>
+> The `default` constraint on `B2.F2` is required to override the unconstrained `A2.F2` with a `T?` parameter. Without it, `T?` in the second override would be interpreted as a nullable value type, so the declaration would not override the unconstrained `A2.F2`. *end example*
 
 An override declaration can access the overridden base method using a *base_access* ([§12.8.15](expressions.md#12815-base-access)).
 

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.
 
@@ -853,12 +853,16 @@
 
 It is a compile-time error for an explicit interface member implementation to include any modifiers ([§15.6](classes.md#156-methods)) other than `extern` or `async`.
 
-An explicit interface method implementation inherits any type parameter constraints from the interface.
+An explicit interface member implementation inherits any type parameter constraints from the interface.
 
-A *type_parameter_constraints_clause* on an explicit interface method implementation 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 signature of the explicit interface method implementation, where `T` is a type parameter, is interpreted as follows:
+A *type_parameter_constraints_clause* on an explicit interface member implementation may only consist of the `class`, `struct`, or `default` *primary_constraint*s. The `class` and `struct` constraints may be placed on *type_parameter*s known according to the inherited constraints to be either reference or non-nullable value types respectively. The `default` constraint may be placed on *type_parameter*s that are not constrained to either reference or value types. Any type of the form `T?` in the signature of the explicit interface member implementation, 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.
+- If a `class` constraint is added for type parameter `T` then `T?` is a nullable reference type.
+- If either a `struct` constraint is added, or no constraint is added and the inherited constraint is a value type constraint, for the type parameter `T` then `T?` is a nullable value type.
+- If a `default` constraint is added for type parameter `T`, the interpretation of `T?` depends on the type argument substituted for `T`:
+  - If `T` is substituted with a reference type `C`, `T?` represents the nullable reference type `C?`.
+  - If `T` is substituted with an already-annotated reference type `C?`, `T?` represents `C?`, not `C??`.
+  - If `T` is substituted with a value type, `T?` represents `T` itself, not `Nullable<T>`.
 
 > *Example*: The following demonstrates how the rules work when type parameters are involved:
 >
@@ -881,6 +885,28 @@
 > Without the type parameter constraint `where T : class`, the base method with the reference-typed type parameter cannot be overridden. *end example*
 <!-- markdownlint-disable MD028 -->
 
+<!-- markdownlint-enable MD028 -->
+> *Example*: The following demonstrates how the `default` constraint works when explicitly implementing an interface method with an unconstrained type parameter:
+>
+> <!-- Example: {template:"standalone-lib-without-using", name:"ExplicitInterfaceMemberImplementations7"} -->
+> ```csharp
+> #nullable enable
+> interface I2
+> {
+>     void F2<T>(T? t) where T : struct;
+>     void F2<T>(T? t);
+> }
+>
+> class C2 : I2
+> {
+>     void I2.F2<T>(T? t) /*where T : struct*/ { }
+>     void I2.F2<T>(T? t) where T : default { }
+> }
+> ```
+>
+> The `default` constraint on `C2.F2` is required to implement the unconstrained `I2.F2` with a `T?` parameter. Without it, `T?` in the second implementation would be interpreted as a nullable value type, so the declaration would not implement the unconstrained `I2.F2`. *end example*
+<!-- markdownlint-disable MD028 -->
+
 <!-- markdownlint-enable MD028 -->
 > *Note*: Explicit interface member implementations have different accessibility characteristics than other members. Because explicit interface member implementations are never accessible through a qualified interface member name in a method invocation or a property access, they are in a sense private. However, since they can be accessed through the interface, they are in a sense also as public as the interface in which they are declared.
 > Explicit interface member implementations serve two primary purposes:

standard/types.md

@@ -1,8 +1,8 @@
 # 8 Types
 
 ## 8.1 General
 
 The types of the C# language are divided into two main categories: ***reference type*** and ***value type***. A value type or a reference type may be a ***generic type***, which takes one or more ***type parameter***s. Type parameters can designate both value types and reference types.
 
 ```ANTLR
 type
@@ -20,8 +20,8 @@
 > *Note*: When a variable is a reference or output parameter, it does not have its own storage but references the storage of another variable. In this case, the ref or out variable is effectively an alias for another variable and not a distinct variable. *end note*
 
 C#’s type system is unified such that *a value of any type can be treated as an object*. Every type in C# directly or indirectly derives from the `object` class type, and `object` is the ultimate base class of all types. Values of reference types are treated as objects simply by viewing the values as type `object`. Values of value types are treated as objects by performing boxing and unboxing operations ([§8.3.13](types.md#8313-boxing-and-unboxing)).
 
 For convenience, throughout this specification, some library type names are written without using their full name qualification. Refer to [§C.5](standard-library.md#c5-library-type-abbreviations) for more information.
 
 ## 8.2 Reference types
 
@@ -737,7 +737,7 @@
 > - A type parameter cannot be used anywhere within an attribute ([§23.2.1](attributes.md#2321-general)).
 > - A type parameter cannot be used in a member access ([§12.8.7](expressions.md#1287-member-access)) or type name ([§7.8](basic-concepts.md#78-namespace-and-type-names)) to identify a static member or a nested type.
 > - A type parameter can only be used as an *unmanaged_type* ([§8.8](types.md#88-unmanaged-types)) if the type parameter is constrained by the unmanaged constraint ([§15.2.5](classes.md#1525-type-parameter-constraints)).
-> - Nullable annotations (`?`) aren’t allowed on an instance of a type parameter unless that type parameter is constrained to be either a reference type or a value type ([§15.2.5](classes.md#1525-type-parameter-constraints)).  
+> - Except when a type parameter is explicitly constrained to value types, the nullable type annotation (`?`) can only be applied to an occurrence of a type parameter when the nullable annotations flag is enabled ([§6.5.9](lexical-structure.md#659-nullable-directive), [§15.2.5](classes.md#1525-type-parameter-constraints)).
 >
 > *end note*