Allowing "fall through" of attributes

[Rufus Smith via Digitalmars-d-learn]

On Tuesday, 19 July 2016 at 17:10:35 UTC, Lodovico Giaretta wrote:
> On Tuesday, 19 July 2016 at 17:05:55 UTC, Rufus Smith wrote:
>> On Tuesday, 19 July 2016 at 16:59:48 UTC, Lodovico Giaretta 
>> wrote:
>>> On Tuesday, 19 July 2016 at 16:50:56 UTC, Rufus Smith wrote:
>>>> On Tuesday, 19 July 2016 at 16:09:38 UTC, Lodovico Giaretta 
>>>> wrote:
>>>>> [...]
>>>>
>>>> But this doesn't create a function with all the attributes 
>>>> of the original? Just one that has the same return type and 
>>>> parameters. What if Fun is pure or extern(C) or some other 
>>>> attributes? I'd like to create a function that is exactly 
>>>> the same in all regards as the original.
>>>
>>> Sorry, I misunderstood your question.
>>> With the method I showed you, if the function is @safe, pure, 
>>> @nogc or nothrow, foo will infer those attributes. But only 
>>> if the operations you do in foo (apart from calling bar) are 
>>> themselves @safe, pure, @nogc or nothrow.
>>> For other things, like extern(C), I don't think there's a 
>>> simple solution; but I'm not an expert, so I hope someone 
>>> else will give you a better answer.
>>
>> What is strange is I cannot even pass an extern(C) function to 
>> foo.
>>
>> void foo(R, A...)(R function(A) bar);
>>
>> extern(C) void bar();
>>
>> foo(&bar)
>>
>> fails. Remove extern and it passes. I have not figured out how 
>> to allow for extern(C) functions to be passed.
>
> That's because an extern function must be called with a 
> different code. So it cannot be cast to a non-extern(C) 
> function pointer, which is what your foo accepts. If you follow 
> my advice, and make the entire function type a parameter of 
> foo, then foo will at least accept your extern(C) function, but 
> it will not be extern(C) itself.

I don't want it to be cast to a non-extern function. What I want 
to do is create the exact same type of function that is passed to 
the template except modify the arguments.

If I use a general parameter for the function, it accepts 
extern(C), but I can't construct a function with it.

mixin("alias F = extern("~functionLinkage!Q~") 
"~(ReturnType!Q).stringof~" 
function"~(Parameters!Q).stringof~";");

gives me a type that looks to be what I want but I can't really 
use it unless I want to declare the function that does the work 
inside foo as mixin string.

One can't do extern(functionLinkage!Q) void baz() to create baz 
with the proper linkage ;/

It looks like I might have to go the mixin way ;/ Going to be 
messy ;/