Passing variables, preserving UDAs: A Gripe

[John Colvin via Digitalmars-d]

On Wednesday, 8 February 2017 at 07:57:15 UTC, Timon Gehr wrote:
> On 07.02.2017 22:59, Nick Sabalausky wrote:
>> Suppose I have some code that operates on a variable's value 
>> and its
>> UDAs. And I want to refactor that code into a reusable 
>> function. Sounds
>> simple enough, right?
>>
>> So, consider a basic example:
>>
>> ----------------------------
>> class Foo
>> {
>>     @("Hello")
>>     string s;
>> }
>>
>> void doStuff(alias var)()
>> {
>>     var = "abc";
>>
>>     import std.traits;
>>     assert(hasUDA!(var, "Hello") == true);
>> }
>>
>> void main()
>> {
>>     @("Hello")
>>     string s;
>>     doStuff!(s);
>>
>>     auto foo = new Foo();
>>     // Error: need 'this' for 'doStuff' of type 'pure nothrow 
>> @nogc
>> @safe void()'
>>     doStuff!(foo.s);
>> }
>> ----------------------------
>>
>> Note the error. Naturally, that cannot compile, because you 
>> can't
>> instantiate a template based on the value of a variable at 
>> runtime (ie,
>> based on the value of `foo`).
>
> It actually can compile. (It just doesn't.)
> There is no essential difference between the two cases.

How much work is it likely to be to make this happen in dmd? I 
imagine your frontend can do this already, but that's not a 
practical solution even for the medium term.