[Proposal]: Task<T> nullability covariance

[333fred]

Task<T> nullability covariance

• Specification: None yet
• Discussion: [Proposal]: `Task<T>` nullability covariance #8996

Summary

Task<T> nullability covariance (LDM tentatively approved, needs design proposal for task-like types)

It should be possible to return a Task<string!> for a Task<string?>.

using System.Threading.Tasks;
#nullable enable
public class C 
{
    public Task<(bool Result, string? Extras)> M() 
    {
        return Task.FromResult((true, "")); // currently produces a warning
    }
}

Design meetings

Split issue from #3868
https://github.com/dotnet/csharplang/blob/main/meetings/2022/LDM-2022-09-28.md#nullability-improvements
https://github.com/dotnet/csharplang/blob/main/meetings/2024/LDM-2024-09-06.md#taskt-nullability-covariance