[Issue 535] New: writef doesn't work on interfaces

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=535

           Summary: writef doesn't work on interfaces
           Product: D
           Version: 0.174
          Platform: PC
        OS/Version: Windows
            Status: NEW
          Severity: normal
          Priority: P2
         Component: Phobos
        AssignedTo: bugzilla at digitalmars.com
        ReportedBy: smjg at iname.com


If writef is fed an argument of an interface type, it just throws a
FormatError.

----------
import std.stdio;

interface Qwert {}

class Yuiop : Qwert {
    char[] toString() {
        return "asdfg";
    }
}

void main() {
    Qwert hjkl = new Yuiop;

    writefln(hjkl);
    writefln("%s", hjkl);
}
----------
Error: std.format formatArg
----------

It should use the object's toString property.  True, interfaces don't extend
Object and so don't have toString as such, but what could be more sensible than
casting it to an Object and then treating it in the same way?

Adding char[] toString() to the interface makes no difference.


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