[Issue 20312] New: Multiple inheritance covariance breaks vtable dispatch

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=20312

          Issue ID: 20312
           Summary: Multiple inheritance covariance breaks vtable dispatch
           Product: D
           Version: D2
          Hardware: x86_64
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: Hexagonalstar64 at gmail.com

Interface covariance causes incorrect vtable lookups when the interfaces have
multiple inheritance as illustrated by this sample code.
``
interface A {
    int x();
}

interface B : A{
    int y();
}

interface C : A {
    int z();
}

class Impl : B, C{
    override:
    int x() { return 0; }
    int y() { return 1; }
    int z() { return 2; }
}


interface GetA{
    A get();
}

interface GetB : GetA{
    B get();
}

interface GetC : GetA {
    C get();
}

class ImplGet : GetB, GetC{
    Impl impl;
    this(Impl impl){
        this.impl = impl;
    }
    override
    Impl get(){
        return impl;
    }
}

import std.stdio;

void main(){
    Impl impl = new Impl();
    C c = impl;

    ImplGet implget = new ImplGet(impl);
    GetC getc = implget;
    C c2 = getc.get;

    writeln(c.z); // prints 2 as expected
    writeln(c2.z); // prints 1 !
}
``

``
$ dmd --version
DMD64 D Compiler v2.088.1
Copyright (C) 1999-2019 by The D Language Foundation, All Rights Reserved
written by Walter Bright
$ dmd bug
$ ./bug
2
1
``

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