alias c=mixin(expr); disallowed, why?

[Timon Gehr]

On 06/23/2013 12:19 PM, Artur Skawina wrote:
> On 06/22/13 21:52, Timothee Cour wrote:
>> Is there a reason the language spec disallows this?
>>
>> ----
>> void main(){
>>     auto a=mixin("1");//OK
>>     alias b=a;//OK
>>     mixin("alias c=a;");//OK
>>     // alias c=mixin("a");//NG : Error: basic type expected, not mixin
>> }
>
> How would that be different from "auto c=mixin("a");"?
>
> It's probably clear, but that error message is misleading, so i'll say
> it anyway - the reason why your 'alias' line does not work is because
> alias requires a symbol, but 'mixin()' is an expression.
> Special-casing mixin-expressions (so that they propagate the symbol when
> that is possible would be a bad idea); the other possibility is to allow
> aliasing /expressions/. But that's a bad idea too, and would likely not
> do what you expect it to do. A mixin-less version could be made to work,
> but there are already other ways to get the same effect.
> Hence the above question.
>
> artur
>

mixin template T(alias x){ }

void foo(){ import std.stdio; writeln("foo"); }

void main(){
     auto a=mixin("1");
     mixin T!a; // ok
     mixin T!(mixin("a")); // ok
     mixin T!1; // ok (!)
     mixin T!(mixin("foo")); // ok (no implicit call)
}