Don't understand, why an implicit conversion of matching generic types doesn't work

[olivermue]

When using FluentResults with generic types like IReadOnlyList<T> it seems that implicit conversation doesn't work and I can't understand why. Maybe someone can enlighten me by this simple example.

What works without any problems is this kind of code:

private async Task<string> FindMatchingItem()
{
    await Task.Delay(1);
    return "1";
}

public async Task<Result<string>> GetMatchingItem()
{
    var item = await FindMatchingItem();
    // Here we can simply return item, which will be automatically be converted into Result<string>
    return item;
}

But, what doesn't work is this one here:

private async Task<IReadOnlyList<string>> FindMatchingItems()
{
    await Task.Delay(1);
    return new List<string> { "1", "2", "3" };
}

public async Task<Result<IReadOnlyList<string>>> GetMatchingItems()
{
    var items = await FindMatchingItems();
    // Doesn't compile. Has to be: return Result.Ok(items);
    return items;
}

The compiler complains with this error message:
CS0266: Cannot implicitly convert type 'System.Collections.Generic.IReadOnlyList<string>' to 'FluentResults.Result<System.Collections.Generic.IReadOnlyList<string>>'. An explicit conversion exists (are you missing a cast?)

If I'm going to change the signature of FindMatchingItems() to return Task<List<string>> the error is gone:

// We changed the return type to be List instead of IReadOnlyList
private async Task<List<string>> FindMatchingItems()
{
    await Task.Delay(1);
    return new List<string> { "1", "2", "3" };
}

// But not here. It is still IReadOnlyList
public async Task<Result<IReadOnlyList<string>>> GetMatchingItems()
{
    var items = await FindMatchingItems();
    return items;
}

So can anyone explain, why List<string> can be converted into Result<IReadOnlyList<string>>, but the very same type IReadOnlyList<string> can't be converted into the desired Result type?