DotNext by Roman Sakno

<PackageReference Include="DotNext" Version="2.5.0" />

 CompletedTask<T, TConstant>

public static class CompletedTask<T, TConstant> where TConstant : Constant<T>
Represents cache of completed tasks.
public static readonly Task<T> Task

Represents the completed task containing a value passed as constant through TConstant generic parameter.