Function pointers/delegates default args were stealth removed?

[Carl Sturtivant]

On Tuesday, 28 August 2012 at 23:34:54 UTC, Carl Sturtivant wrote:
> 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.

Another, more conservative, approach would be for the compiler to 
regard the non-general functions in such a group created by my 
transformation to be always inlined, so the default parameters 
are always evaluated before the call. Then your interesting 
example would work the way you expect.