#nullable enable

using System;

using System.Collections.Generic;

using System.Linq;

using System.Diagnostics.CodeAnalysis;

public class Program

{

    public static void Main()

    {

        string? s1 = GetValueOrDefault<string>(null);

        string s2 = GetValueOrDefault<string>("");   // works, C# realizes that the return value can't be null

        IEnumerable<string?> e1 = GetValuesOrDefault<string>(null);

        IEnumerable<string> e2 = GetValuesOrDefault<string>("");   // I want to get rid of the warning here

    }

    [return: NotNullIfNotNull(nameof(defaultValue))]

    public static T? GetValueOrDefault<T>(T? defaultValue)

    { 

        return GetValueFromSomewhere<T>() ?? defaultValue;

    }

    public static IEnumerable<T?> GetValuesOrDefault<T>(T? defaultValue)

    {

        return GetValuesFromSomewhere<T>().Select(x => x ?? defaultValue);

    }

    // Dummy placeholders for my MCVE. My real methods read values from my DB repository.

    private static T? GetValueFromSomewhere<T>() => default(T);

    private static IEnumerable<T?> GetValuesFromSomewhere<T>() => new T?[] { default(T) };

}