Default Interface Implementation: Unexpected behavior at runtime

[AlekseyTs]

CC @yizhang82

Compile and run the following code:

public interface I1<T>
{
    void M1()
    {
        System.Console.WriteLine("I1.M1");
    }
    void M2<S>()
    {
        System.Console.WriteLine("I1.M2");
    }
}

public interface I2<T> : I1<T>
{
    void I1<T>.M1() 
    {
        System.Console.WriteLine("I2.I1.M1");
    }
    void I1<T>.M2<S>() 
    {
        System.Console.WriteLine("I2.I1.M2");
    }
}

public interface I3<T> : I1<T>
{}

class Test1 : I1<int>
{
    static void Main()
    {
        I1<int> i1Int = new Test1();
        i1Int.M1();
        i1Int.M2<int>();
        I1<long> i1Long = new Test2();
        i1Long.M1();
        i1Long.M2<int>();
    }
}
class Test2 : I2<long>
{}

Observed:

I1.M1
I1.M2
I1.M1
I1.M2

Expected:

I1.M1
I1.M2
I2.I1.M1
I2.I1.M2

Here is the IL for the assembly:

//  Microsoft (R) .NET Framework IL Disassembler.  Version 4.6.1055.0
//  Copyright (c) Microsoft Corporation.  All rights reserved.



// Metadata version: v4.0.30319
.assembly extern System.Runtime
{
  .publickeytoken = (B0 3F 5F 7F 11 D5 0A 3A )                         // .?_....:
  .ver 4:1:0:0
}
.assembly extern System.Console
{
  .publickeytoken = (B0 3F 5F 7F 11 D5 0A 3A )                         // .?_....:
  .ver 4:0:0:0
}
.assembly debug7
{
  .custom instance void [System.Runtime]System.Runtime.CompilerServices.CompilationRelaxationsAttribute::.ctor(int32) = ( 01 00 08 00 00 00 00 00 ) 
  .custom instance void [System.Runtime]System.Runtime.CompilerServices.RuntimeCompatibilityAttribute::.ctor() = ( 01 00 01 00 54 02 16 57 72 61 70 4E 6F 6E 45 78   // ....T..WrapNonEx
                                                                                                                   63 65 70 74 69 6F 6E 54 68 72 6F 77 73 01 )       // ceptionThrows.

  // --- The following custom attribute is added automatically, do not uncomment -------
  //  .custom instance void [System.Runtime]System.Diagnostics.DebuggableAttribute::.ctor(valuetype [System.Runtime]System.Diagnostics.DebuggableAttribute/DebuggingModes) = ( 01 00 07 01 00 00 00 00 ) 

  .hash algorithm 0x00008004
  .ver 0:0:0:0
}
.module debug7.exe
// MVID: {2D9B8221-4878-4FA1-AEA5-814FD96D8095}
.imagebase 0x00400000
.file alignment 0x00000200
.stackreserve 0x00100000
.subsystem 0x0003       // WINDOWS_CUI
.corflags 0x00000001    //  ILONLY
// Image base: 0x01210000


// =============== CLASS MEMBERS DECLARATION ===================

.class interface public abstract auto ansi I1`1<T>
{
  .method public hidebysig newslot virtual 
          instance void  M1() cil managed
  {
    // Code size       13 (0xd)
    .maxstack  8
    IL_0000:  nop
    IL_0001:  ldstr      "I1.M1"
    IL_0006:  call       void [System.Console]System.Console::WriteLine(string)
    IL_000b:  nop
    IL_000c:  ret
  } // end of method I1`1::M1

  .method public hidebysig newslot virtual 
          instance void  M2<S>() cil managed
  {
    // Code size       13 (0xd)
    .maxstack  8
    IL_0000:  nop
    IL_0001:  ldstr      "I1.M2"
    IL_0006:  call       void [System.Console]System.Console::WriteLine(string)
    IL_000b:  nop
    IL_000c:  ret
  } // end of method I1`1::M2

} // end of class I1`1

.class interface public abstract auto ansi I2`1<T>
       implements class I1`1<!T>
{
  .method private hidebysig virtual final 
          instance void  'I1<T>.M1'() cil managed
  {
    .override  method instance void class I1`1<!T>::M1()
    // Code size       13 (0xd)
    .maxstack  8
    IL_0000:  nop
    IL_0001:  ldstr      "I2.I1.M1"
    IL_0006:  call       void [System.Console]System.Console::WriteLine(string)
    IL_000b:  nop
    IL_000c:  ret
  } // end of method I2`1::'I1<T>.M1'

  .method private hidebysig virtual final 
          instance void  'I1<T>.M2'<S>() cil managed
  {
    .override  method instance void class I1`1<!T>::M2<[1]>()
    // Code size       13 (0xd)
    .maxstack  8
    IL_0000:  nop
    IL_0001:  ldstr      "I2.I1.M2"
    IL_0006:  call       void [System.Console]System.Console::WriteLine(string)
    IL_000b:  nop
    IL_000c:  ret
  } // end of method I2`1::'I1<T>.M2'

} // end of class I2`1

.class interface public abstract auto ansi I3`1<T>
       implements class I1`1<!T>
{
} // end of class I3`1

.class private auto ansi beforefieldinit Test1
       extends [System.Runtime]System.Object
       implements class I1`1<int32>
{
  .method private hidebysig static void  Main() cil managed
  {
    .entrypoint
    // Code size       42 (0x2a)
    .maxstack  1
    .locals init (class I1`1<int32> V_0,
             class I1`1<int64> V_1)
    IL_0000:  nop
    IL_0001:  newobj     instance void Test1::.ctor()
    IL_0006:  stloc.0
    IL_0007:  ldloc.0
    IL_0008:  callvirt   instance void class I1`1<int32>::M1()
    IL_000d:  nop
    IL_000e:  ldloc.0
    IL_000f:  callvirt   instance void class I1`1<int32>::M2<int32>()
    IL_0014:  nop
    IL_0015:  newobj     instance void Test2::.ctor()
    IL_001a:  stloc.1
    IL_001b:  ldloc.1
    IL_001c:  callvirt   instance void class I1`1<int64>::M1()
    IL_0021:  nop
    IL_0022:  ldloc.1
    IL_0023:  callvirt   instance void class I1`1<int64>::M2<int32>()
    IL_0028:  nop
    IL_0029:  ret
  } // end of method Test1::Main

  .method public hidebysig specialname rtspecialname 
          instance void  .ctor() cil managed
  {
    // Code size       8 (0x8)
    .maxstack  8
    IL_0000:  ldarg.0
    IL_0001:  call       instance void [System.Runtime]System.Object::.ctor()
    IL_0006:  nop
    IL_0007:  ret
  } // end of method Test1::.ctor

} // end of class Test1

.class private auto ansi beforefieldinit Test2
       extends [System.Runtime]System.Object
       implements class I2`1<int64>,
                  class I1`1<int64>
{
  .method public hidebysig specialname rtspecialname 
          instance void  .ctor() cil managed
  {
    // Code size       8 (0x8)
    .maxstack  8
    IL_0000:  ldarg.0
    IL_0001:  call       instance void [System.Runtime]System.Object::.ctor()
    IL_0006:  nop
    IL_0007:  ret
  } // end of method Test2::.ctor

} // end of class Test2


// =============================================================

[yizhang82]

@AlekseyTs Thanks for reporting this. Most likely GVMs are not handled correctly today...

[yizhang82]

This is addressed by dotnet/coreclr#12753