Why are C # 7.2 features not compiling in a UWP application?

Question

Why are C # 7.2 features not compiling in a UWP application?

... in particular, the parameters in( readonly ref). Here is my situation:

I have a UWP project and a UWP Unit Test project in the same Visual Studio solution. Both projects focus on C # 7.2. The main UWP project has this class (pay attention to the parameters in):

public struct Cell
{
    public Cell(int x, int y)
    {
        X = x;
        Y = y;
    }

    public int X { get; }

    public int Y { get; }

    public static Cell operator +(in Cell left, in Cell right)
    {
        return new Cell(left.X + right.X, left.Y + right.Y);
    }

    public static Cell operator -(in Cell left, in Cell right)
    {
        return new Cell(left.X - right.X, left.Y - right.Y);
    }


    public override string ToString() => $"{X}, {Y}";
}

When I use those statements from the UWP test project :

    [TestMethod]
    public void TestMethod1()
    {
        Cell cell1 = new Cell(0, 0);
        Cell cell2 = new Cell(1, 1);

        var added = cell1 + cell2 ;
        var minus = cell1 - cell2 ;
    }

I get:

UnitTest.cs(16,25,16,38): error CS0019: Operator '+' cannot be applied to operands of type 'Cell' and 'Cell'
UnitTest.cs(17,25,17,38): error CS0019: Operator '-' cannot be applied to operands of type 'Cell' and 'Cell'