[Version 9.0] Feature support for unconstrained type parameter annotations

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
@@ -409,6 +409,7 @@
     | 'struct'
     | 'notnull'
     | 'unmanaged'
+    | 'default'
     ;
 
 secondary_constraint
@@ -429,7 +430,9 @@
 
 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 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 value type.
 
 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.
 
@@ -506,15 +509,15 @@
 >     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
@@ -533,6 +536,8 @@
 
 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 allows a *nullable_type_annotation* on a type parameter that is not constrained to reference types or value types.
+
 Pointer types are never allowed to be type arguments, and don’t 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).
@@ -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
@@ -4577,7 +4582,7 @@
     : '!'
     ;
 
 overloadable_unary_operator
     : '+' | '-' | logical_negation_operator | '~' | '++' | '--' | 'true' | 'false'
     ;