[Issue 10971] New: assigning derived class to base should not trigger promotion of result

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=10971

           Summary: assigning derived class to base should not trigger
                    promotion of result
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: DMD
        AssignedTo: nobody at puremagic.com
        ReportedBy: monarchdodra at gmail.com


--- Comment #0 from monarchdodra at gmail.com 2013-09-05 06:16:03 PDT ---
EG:

//----
class A
{
    A next;
}
class B : A
{}

void main()
{
    B first = new B;
    B last = first;
    //foreach(_ ; 0 .. 10)
        last = (last.next = new B); [1]
}
//----
[1]: Error: cannot implicitly convert expression (last.next = new B) of type
main.A to main.B

When the call the compiler should be able to type the result of "last.next =
new B" as a "B" (the type of "new B"), and not an "A" (the type of
"last.next").

Other usecase:
//----
class A
{
    A next;
}
class B : A
{}

void foo(B){}

void main()
{
    A a;
    foo(a = new B);
}
//----
Error: function main.foo (B _param_0) is not callable using argument types (A)

The workaround is trivial of course, but I'd have expect the compiler could
handle it.

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