[Mono-bugs] [Bug 408516] New: doubt on allowed implicit conversion from a generic argument instantiated as an interface

[bugzilla_noreply at novell.com]

https://bugzilla.novell.com/show_bug.cgi?id=408516


           Summary: doubt on allowed implicit conversion from a generic
                    argument instantiated as an interface
           Product: Mono: Compilers
           Version: SVN
          Platform: Other
        OS/Version: Other
            Status: NEW
          Severity: Normal
          Priority: P5 - None
         Component: C#
        AssignedTo: mono-bugs at lists.ximian.com
        ReportedBy: atsushi at ximian.com
         QAContact: mono-bugs at lists.ximian.com
                CC: mprobst at novell.com, msafar at novell.com
          Found By: ---


(Copying other's report here: http://d.hatena.ne.jp/NyaRuRu/20080711/p2 )

In the last line from the following code, it tries to convert from IList<int>
to generic class Hoge<IList<int>>. gmcs allows it, but csc rejects it (CS0266).

-------
using System;
using System.Collections;
using System.Collections.Generic;

public class Hoge<T>
{
        public static implicit operator Hoge<T>(T value)
        {
                return null;
        }
}

public class Test
{
        public void Foo ()
        {
                // OK
                {
                        Hoge<List<int>> hoge = new List<int>();
                }
                // OK
                {
                        Hoge<IList<int>> hoge = new List<int>();
                }
                // NG
                {
                        IList<int> x = new List<int> ();
                        Hoge<IList<int>> hoge = x;
                }
        }
}

--------

csc reports:

test.cs(29,26): error CS0266: Cannot implicitly convert type
        'System.Collections.Generic.IList<int>' to
        'Hoge<System.Collections.Generic.IList<int>>'. An explicit conversion
        exists (are you missing a cast?)

Generally, implicit conversion from an interface is prohibited (CS0552), and if
it is practically due to JIT efficiency, it is sort of reasonable that csc
rejects such a conversion above (probably for the purpose of generic code
sharing).

Since it smells like a bug in csc (the original reporter rather thinks so,
while I doubt in gmcs), it is posted as a feedback to Microsoft:
https://connect.microsoft.com/VisualStudioJapan/feedback/ViewFeedback.aspx?FeedbackID=355863


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