Function pointers/delegates default args were stealth removed?

[Carl Sturtivant]

On Tuesday, 28 August 2012 at 21:40:01 UTC, Timon Gehr wrote:
> On 08/28/2012 10:33 PM, Carl Sturtivant wrote:
>> On Monday, 27 August 2012 at 00:44:54 UTC, Walter Bright wrote:
>>> On 8/26/2012 4:50 PM, Timon Gehr wrote:
>>>> On 08/27/2012 12:41 AM, Walter Bright wrote:
>>>>>
>>>>> The trouble for function pointers, is that any default args 
>>>>> would need
>>>>> to be part of the type, not the declaration.
>>>>>
>>>>
>>>> They could be made part of the variable declaration.
>>>
>>> You mean part of the function pointer variable?
>>>
>>> Consider what you do with a function pointer - you pass it to 
>>> someone
>>> else. That someone else gets it as a type, not a declaration. 
>>> I.e. you
>>> lose the default argument information, since that is not 
>>> attached to
>>> the type.
>>
>> I think this is the right behavior too. Default arguments are 
>> IMHO just
>> a compact way to write some simply related overloaded 
>> functions, e.g. thus:
>>
>>   int sum(int x, int y = 1 ) { return x + y; }
>>
>> is just a compact way to write
>>
>>   int sum(int x, int y) { return x + y; }
>>   int sum(int x) { return sum(x, 1); }
>> ...
>
> This interpretation is simply wrong.
>
> import std.stdio, std.c.stdlib;
>
> void* alloca20bytes(void* x = alloca(20)){ return x; }
>
> // your suggested transformation:
>
> /+void* alloca20bytes(void* x){ return x; }
> void* alloca20bytes(){ return alloca20bytes(alloca(20)); }+/
>
> // would break the caller:
>
> void main(){
>     auto x = (cast(int*)alloca20bytes())[0..5];
>     x[] = 0;
>     x[] += 2;
>     writeln(x);
> }

Function inlining or not in the presence of alloca calls and 
similar using the existing stack frame are problematic. If the 
first call was inlined by the compiler, that would un-break the 
"problem". I suggest that we simply define default arguments via 
the transformation I suggested, and regard
   void* alloca20bytes(void* x = alloca(20)){ return x; }
as broken. It's not compelling for a lot of reasons.

Of course there may be something else wrong with the 
transformation! Fire away.