[Mono-bugs] [Bug 569827] New: Multiple overloads of an extension method can be incorrectly marked as ambiguous

[bugzilla_noreply at novell.com]

http://bugzilla.novell.com/show_bug.cgi?id=569827

http://bugzilla.novell.com/show_bug.cgi?id=569827#c0


           Summary: Multiple overloads of an extension method can be
                    incorrectly marked as ambiguous
    Classification: Mono
           Product: Mono: Compilers
           Version: SVN
          Platform: Macintosh
        OS/Version: Mac OS X 10.6
            Status: NEW
          Severity: Normal
          Priority: P5 - None
         Component: C#
        AssignedTo: mono-bugs at lists.ximian.com
        ReportedBy: dmitchell at logos.com
         QAContact: mono-bugs at lists.ximian.com
          Found By: ---
           Blocker: ---


Created an attachment (id=336004)
 --> (http://bugzilla.novell.com/attachment.cgi?id=336004)
A copy of the code listing from the description.

User-Agent:       Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10_6_2; en-us)
AppleWebKit/531.21.8 (KHTML, like Gecko) Version/4.0.4 Safari/531.21.10

Consider the following C# program:

===

using System;

namespace MonoBugs
{
    public struct Foo<T>
    {
        public T Item;
    }

    public static class Bar
    {
        public static void DoStuff<T>(this T item, Action<T> fn)
            where T : class
        {
            Console.WriteLine("First overload");
            if (item != null)
                fn(item);
        }

        public static void DoStuff<T>(this T? item, Action<T> fn)
            where T : struct
        {
            Console.WriteLine("Second overload");
            if (item.HasValue)
                fn(item.Value);
        }
    }

    public static class Program
    {
        public static void Main()
        {
            Foo<int>? value = new Foo<int> { Item = 3 };
            value.DoStuff(x => Console.WriteLine(x.Item));
        }
    }
}

===

Under Microsoft's .NET implementation, this code compiles and runs just fine,
correctly selecting the second overload of DoStuff when it is invoked on the
second line of Main().

Mono, on the other hand, incorrectly fails to compile with an error message
similar to

foo.cs(39,31): error CS0121: The call is ambiguous between the following
methods or properties: `MonoBugs.Bar.DoStuff<MonoBugs.Foo<int>>(this
MonoBugs.Foo<int>?, System.Action<MonoBugs.Foo<int>>)' and
`MonoBugs.Bar.DoStuff<MonoBugs.Foo<int>?>(this MonoBugs.Foo<int>?,
System.Action<MonoBugs.Foo<int>?>)'

This probably happens either because Nullable<Foo<int>>? is being treated as a
class for the purpose of generic constraint checking or because generic
constraints are not being fully checked before the compiler gives up.

Reproducible: Always

Steps to Reproduce:
1. Take the program listing from the details and put it in a file
2. Compile the file with gmcs
3.
Actual Results:  
The compiler fails with a message similar to

foo.cs(39,31): error CS0121: The call is ambiguous between the following
methods or properties: `MonoBugs.Bar.DoStuff<MonoBugs.Foo<int>>(this
MonoBugs.Foo<int>?, System.Action<MonoBugs.Foo<int>>)' and
`MonoBugs.Bar.DoStuff<MonoBugs.Foo<int>?>(this MonoBugs.Foo<int>?,
System.Action<MonoBugs.Foo<int>?>)'

Expected Results:  
The compiler should emit a program that correctly calls the second overload of
DoStuff.

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