C# 4.0 Compiler Crash

Question

This code sample is not able to be compiled. Any work arounds out there?

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication1
{
    using church = Func<dynamic, dynamic, dynamic>;

    class Program
    {
        static void Main(string[] args)
        {
            church True = (a, b) => a;
            church False = (a, b) => b;

            Func<church, church, church> And = (x, y) => x(y(True, False), False);
        }
    }
}

Error 6 Internal Compiler Error (0xc0000005 at address 5476A4CC): likely culprit is 'EMITIL'. An internal error has occurred in the compiler. To work around this problem, try simplifying or changing the program near the locations listed below. Locations at the top of the list are closer to the point at which the internal error occurred. Errors such as this can be reported to Microsoft by using the /errorreport option. TestApplication

Answer 1

Clearly that is a compiler bug.

I mentioned this to one of our testers and he says:

I’m happy to report that this has already been fixed and you’ll see this fix in the next version of VS. You’ll actually see it fixed in the BUILD Developer Preview for Visual Studio as well!

Apologies for the error, and thanks for bringing this to our attention.

Answer 2

I reproduced the crash using VS2010 (WinXP 64).

Two workarounds:

1. don't use the using alias

The following code compiles cleanly on VS2010:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            Func<dynamic, dynamic, dynamic> True = (a, b) => a;
            Func<dynamic, dynamic, dynamic> False = (a, b) => b;

            Func<Func<dynamic, dynamic, dynamic>, 
                 Func<dynamic, dynamic, dynamic>,
                 Func<dynamic, dynamic, dynamic> > And 
                = (x, y) => x(y(True, False), False);
        }
    }
}

2. use the Mono compiler

No problem with Mono 2.10 compiler (dmcs).

[mono] /tmp @ dmcs test.cs
test.cs(18,42): warning CS0219: The variable `And' is assigned but its value is never used
Compilation succeeded - 1 warning(s)
[mono] /tmp @ ./test.exe 
[mono] /tmp @ 

This was tested on linux.

1. You can run binaries created with mono on Windows .NET.
2. Mono compiler comes with an installer MSI and runs on Windows as well.

Answer 3

EDIT: I've now managed to reproduce it, and I have a potential workaround.

This works:

csc Test.cs

This doesn't:

csc /debug+ Test.cs

So it looks like it's a problem with the debug information. If you can live without that in your particular scenario, you're good to go...

EDIT: I've just tested, and this happens with /debug:pdbonly as well...

EDIT: Just in case anyone was wondering, I'll ping Eric Lippert about this.

EDIT: This is now the minimal repro I've found:

using church = System.Func<dynamic>;

class Program
{
    static void Main() {}
}

Answer 4

Here's another workaround: don't use Func, use a good old delegate type.

public delegate dynamic Church(dynamic x, dynamic y);

class Program {
    static void Main(string[] args) {
        Church True = (a, b) => a;
        Church False = (a, b) => b;

        Func<Church, Church, Church> And = (x, y) => x(y(True, False), False);
    }
}

This also has the benefit, that Church is defined everywhere, and not just as a per-file using alias.