[Issue 2050] interfaces should allow final methods with body

[Robert Fraser]

d-bugmail at puremagic.com wrote:
> http://d.puremagic.com/issues/show_bug.cgi?id=2050
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> ------- Comment #2 from andrei at metalanguage.com  2008-04-28 02:21 -------
> (In reply to comment #1)
>> That would open the door to the dreaded diamond pattern.
>>
>> You'd just have the compiler generate ambiguity errors in those cases I
>> suppose?
> 
> The diamond pattern can't occur in schemes with single inheritance of
> implementation.

That's true, but the purpose of interfaces is to allow for multiple 
inheritance in a safe way. Say you have two interfaces, IZombie from 
Library A and IPirate from Library B. You have a class ZombiePirate that 
implements both these interfaces. Right now, IZombie has a final method 
"dance()", and IPirate has the dance() method as virtual. All is well, 
since a call to an instance's dance() method would result in 
IZombie.dance()'s implementation being called. But if IPirate suddenly 
got a dance() implementation, your code would break simply by updating 
Library B.