Address issue #1246 (#1258)

Adds `notnull` as a contextual keyword and defines
how the ambiguity with *class_type* is resolved.

Co-authored-by: Nigel-Ecma <[email protected]>

Author: Nigel-Ecma

standard/classes.md

@@ -493,14 +493,16 @@ For a type parameter `T` when the type argument is a nullable reference type `C?
 
 The ***not null*** constraint specifies that a type argument used for the type parameter should be a non-nullable value type or a non-nullable reference type. A type argument that isn’t a non-nullable value type or a non-nullable reference type is allowed, but a compiler may produce a diagnostic warning.
 
+Because `notnull` is not a keyword, in *primary_constraint* the not null 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 `notnull` succeeds it is treated as a `class_type`. Otherwise it is treated as the not null constraint.
+
 The value type constraint specifies that a type argument used for the type parameter shall be a non-nullable value type. All non-nullable struct types, enum types, and type parameters having the value type constraint satisfy this constraint. Note that although classified as a value type, a nullable value type ([§8.3.12](types.md#8312-nullable-value-types)) does not satisfy the value type constraint. A type parameter having the value type constraint shall not also have the *constructor_constraint*, although it may be used as a type argument for another type parameter with a *constructor_constraint*.
 
 > *Note*: The `System.Nullable<T>` type specifies the non-nullable value type constraint for `T`. Thus, recursively constructed types of the forms `T??` and `Nullable<Nullable<T>>` are prohibited. *end note*
 
-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 unmanaged type constraint specifies that a type argument used for the type parameter shall be a non-nullable unmanaged type ([§8.8](types.md#88-unmanaged-types)).
 
+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.
+
 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).

standard/lexical-structure.md

@@ -591,12 +591,12 @@ A ***contextual keyword*** is an identifier-like sequence of characters that has
 
 ```ANTLR
 contextual_keyword
-    : 'add'    | 'alias'      | 'ascending' | 'async'     | 'await'
-    | 'by'     | 'descending' | 'dynamic'   | 'equals'    | 'from'
-    | 'get'    | 'global'     | 'group'     | 'into'      | 'join'
-    | 'let'    | 'nameof'     | 'on'        | 'orderby'   | 'partial'
-    | 'remove' | 'select'     | 'set'       | 'unmanaged' | 'value'
-    | 'var'    | 'when'       | 'where'     | 'yield'
+    : 'add'     | 'alias'      | 'ascending' | 'async'     | 'await'
+    | 'by'      | 'descending' | 'dynamic'   | 'equals'    | 'from'
+    | 'get'     | 'global'     | 'group'     | 'into'      | 'join'
+    | 'let'     | 'nameof'     | 'notnull'   | 'on'        | 'orderby'
+    | 'partial' | 'remove'     | 'select'    | 'set'       | 'unmanaged'
+    | 'value'   | 'var'        | 'when'      | 'where'     | 'yield'
     ;
 ```