2x1=10

Update:
I opened a bug tick­et for the described issue and it got fixed with­in the week.

Orig­i­nal post:
At work we recent­ly stum­bled across an inter­est­ing prob­lem: While com­pi­la­tion of our C# 6 code base would work fine on Win­dows, a guy that works on OSX using Xam­arin Stu­dio sud­den­ly was unable to com­pile the code, because “Error: Com­pil­er crashed with code 1”. No line num­ber, no noth­ing, only that.

After a run through git bisect we even­tu­al­ly were able to track down the com­mit where I found the error to lie with­in an await hap­pen­ing in a string inter­po­la­tion. Sad­ly, this prob­lem was intro­duced by sim­ply (and auto­mat­i­cal­ly) refac­tor­ing all occur­rences of String.Format() in the file.

I was able to repro­duce the prob­lem with the fol­low­ing sim­ple code:

private async Task<int> Foo() 
{
    await Task.Delay(1);
    return 42;
}

private async Task Bar()
{
    Console.WriteLine($"Something {await Foo()}");
}

Appar­ent­ly, the mono com­pil­er doesn’t like that too much. The solu­tion to this is sim­ple: Don’t await with­in the inter­po­lat­ed string.
Instead, do it out­side:

private async Task Bar()
{
    var foo = await Foo();
    Console.WriteLine($"Something {foo}");
}

And that’s that.