Fixes #879

Clarify the required accessibility of any task builder type and its required members.

Note that we will need to update this text once dotnet/roslyn#71283 is fixed and merged.

standard/classes.md

@@ -5354,13 +5354,13 @@ Task types can vary in their exact definition, but from the language’s point o
 >
 > *end example*
 
-A task builder type is a class or struct type that corresponds to a specific task type ([§15.15.2](classes.md#15152-task-type-builder-pattern)).
+A task builder type is a class or struct type that corresponds to a specific task type ([§15.15.2](classes.md#15152-task-type-builder-pattern)). The task builder type must have the same declared accessibility as its corresponding task type.
 
 An async function has the ability to suspend evaluation by means of await expressions ([§12.9.8](expressions.md#1298-await-expressions)) in its body. Evaluation may later be resumed at the point of the suspending await expression by means of a ***resumption delegate***. The resumption delegate is of type `System.Action`, and when it is invoked, evaluation of the async function invocation will resume from the await expression where it left off. The ***current caller*** of an async function invocation is the original caller if the function invocation has never been suspended or the most recent caller of the resumption delegate otherwise.
 
 ### 15.15.2 Task-type builder pattern
 
-A task builder type can have at most one type parameter and cannot be nested in a generic type. A task builder type shall have the following accessible members (for non-generic task builder types, `SetResult` has no parameters):
+A task builder type can have at most one type parameter and cannot be nested in a generic type. A task builder type shall have the following members (for non-generic task builder types, `SetResult` has no parameters) with declared `public` accessibility:
 
 ```csharp
 class «TaskBuilderType»<T>
@@ -5382,7 +5382,7 @@ class «TaskBuilderType»<T>
     public «TaskType»<T> Task { get; }
 }
 ```
-  
+
 The compiler generates code that uses the «TaskBuilderType» to implement the semantics of suspending and resuming the evaluation of the async function. The compiler uses the «TaskBuilderType» as follows:
 
 - `«TaskBuilderType».Create()` is invoked to create an instance of the «TaskBuilderType», named `builder` in this list.