UDA and ReturnType!(__traits...) doesn't work

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On Sunday, 3 May 2015 at 17:22:00 UTC, filcuc wrote:
> Hi all,
> i'm working in the generation of the code but i'm failing in 
> extracting a function return type when invoking the 
> ReturnType!(T) type trait and mixing it with 
> __traits(getMember) function.
> Can anyone help me? or explaining what is happenning?
> i've created a gist here: 
> https://gist.github.com/filcuc/14c3a6cac89beb69cccd
>
> The error message is the following:
>
> main.d-mixin-58(58): Error: template instance 
> main.Person.ReturnType!(name) does not match template 
> declaration ReturnType(func...) if (func.length == 1 && 
> isCallable!func)
> main.d-mixin-58(58): Error: template instance 
> std.traits.ReturnType!(Monitor) does not match template 
> declaration ReturnType(func...) if (func.length == 1 && 
> isCallable!func)
> main.d(73): Error: template instance main.IterateUDA!(Person) 
> error instantiating

Try adding the following around your 'string returnType = ...' 
code:

static if(isSomeFunction!(__traits(get member, T, member))
{
     ...
}

The problem is that you're looping over all the fields in T, 
including the data members which obviously don't have a return 
type. I guess ReturnType should probably use a static assert to 
give a better error message.