Why do C# 7.2 features not compile in a UWP application?

Question

... in particular, in (readonly ref) parameters. Here's my situation:

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

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 operators 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'

However, the same usage inside the main UWP project does not produce any compiler errors.

Why is that?

Answer 1

There is a compiler bug with in in operators, which gets lost when loading the operator from another project/assembly.

https://github.com/dotnet/roslyn/pull/23508 (the fix will ship in 15.6 preview 3)

https://github.com/dotnet/roslyn/issues/23689 (another report of this issue)